Ink jet ink, ink cartridge, and ink jet recording method

ABSTRACT

An ink jet ink contains a polyurethane polymer and a self-dispersing pigment. The polyurethane polymer has units derived from a polyisocyanate, a polyol having no acid group, and a diol having an acid group. The molar ratio of the percentage of the urethane bond in the polyurethane polymer to the percentage of the urea bond in the polyurethane polymer is 85.0/15.0 or more and 100.0/0 or less. The ink jet ink may be capable of producing an image having high optical density, scratch resistance, and highlighter resistance, and which has excellent ink ejection stability and can reduce image deflection caused by face wetting.

TECHNICAL FIELD

The present invention relates to an ink jet ink, an ink cartridgecontaining the ink jet ink, and an ink jet recording method.

BACKGROUND ART

In recent years, various important characteristics have been requiredfor ink jet inks, for example, ink reliability (including ejectionstability and ejection accuracy), image quality (including high opticaldensity), and image fastness (including scratch resistance andhighlighter resistance). In order to improve these characteristics,various pigment inks containing a polyurethane polymer have been studied(PTLs 1 to 4). PTL 1 discloses an aqueous ink jet ink that contains adispersion of a polyurethane polymer having an acid group and aself-dispersing pigment. PTL 2 discloses a pigment dispersion thatcontains a polyurethane polymer, which contains a compound having threeor more hydroxy groups and a soft segment having a carboxy group. Thesoft segment contains a polyol. PTL 3 discloses an aqueous pigment inkcontaining a polyurethane polymer in which the ratio of the urethanebond to the urea bond is specified. PTL 4 discloses an aqueous pigmentink that contains a polyurethane polymer having no urea bond.

CITATION LIST Patent Literature

-   PTL 1: PCT Japanese Translation Patent Publication No. 2005-515289-   PTL 2: Japanese Patent Laid-Open No. 2008-179657-   PTL 3: Japanese Patent Laid-Open No. 9-291242-   PTL 4: Japanese Patent Laid-Open No. 2004-285344

SUMMARY OF INVENTION Technical Problem

The present inventors found that conventional pigment inks containing apolyurethane polymer have improved ink reliability, image quality, andimage fastness but still do not sufficiently satisfy the requirements.

The present inventors found that use of the polyurethane polymerdescribed in PTL 1 results in an acid value as low as approximately 20to 30 mgKOH/g and consequently poor ink ejection stability. It was alsofound that a polyurethane polymer that contains 10% by mole or more of acompound having three or more hydroxy groups based on all the compoundshaving a hydroxy group as disclosed in PTL 2 becomes excessively rigid,resulting in poor image fastness. It was also found that thepolyurethane polymer disclosed in PTL 2 has an acid value as low as 30mgKOH/g, resulting in poor ink ejection stability.

In the polyurethane polymer disclosed in PTL 3, the molar ratio of theurethane bond to the urea bond is in the range of 80/20 to 50/50. Thepresent inventors found that the ejection of such an ink containing apolyurethane polymer as disclosed in PTL 3 through an ink jet recordinghead results in the deposition of a polymer component of the ink on asurface of the recording head on which the ejection ports are disposed(hereinafter referred to as a “face”). This phenomenon is referred to as“face wetting”. The polymer component deposited on the face may changethe intended flight direction of an ejected ink droplet. This deflectionmay be small immediately after the ejection of the ink droplet butincrease before reaching a recording medium, causing a problem of thedeflection of some dots in the image (hereinafter referred to as “imagedeflection”). This image deflection occurs not only in a thermal ink jetmethod but also in an ink jet method involving the use of apiezoelectric element.

The polyurethane polymer disclosed in PTL 4 has no urea bond. Thus, theaqueous pigment ink disclosed in PTL 4 rarely causes image deflection.However, because of the use of a polymer-dispersed pigment, a desiredoptical density cannot be achieved.

Accordingly, the present invention provides an ink jet ink that producesan image having high optical density, scratch resistance, andhighlighter resistance, has excellent ink ejection stability, and canreduce image deflection caused by face wetting. The present inventionalso provides an ink cartridge containing an ink according to anembodiment of the present invention and an ink jet recording method.

Solution to Problem

These objects can be achieved by the present invention described below.An ink jet ink according to one aspect of the present invention containsa polyurethane polymer and a self-dispersing pigment. The polyurethanepolymer has units derived from a polyisocyanate, a polyol having no acidgroup, and a diol having an acid group. The polyol having no acid groupcontains a polyether polyol having no acid group, and the percentage (%by mole) constituted by a unit derived from the polyether polyol havingno acid group with respect to all the units derived from the polyolhaving no acid group is 80% by mole or more. The polyether polyol has anumber-average molecular weight of 450 or more and 4,000 or less. Theratio of the percentage (% by mole) of urethane bonds in thepolyurethane polymer to the percentage (% by mole) of urea bonds in thepolyurethane polymer is 85.0/15.0 or more and 100.0/0 or less. The diolhaving an acid group is at least one selected from dimethylolpropionicacid and dimethylolbutanoic acid, and the acid value of the polyurethanepolymer resulting from the unit(s) derived from the diol having an acidgroup is 40 mgKOH/g or more and 140 mgKOH/g or less.

Advantageous Effects of Invention

The present invention can provide an ink jet ink that produces an imagehaving high optical density, scratch resistance, and highlighterresistance, has excellent ink ejection stability, and can reduce imagedeflection caused by face wetting. The present invention can alsoprovide an ink cartridge containing an ink according to an embodiment ofthe present invention and an ink jet recording method.

DESCRIPTION OF EMBODIMENTS

The present invention will be described in detail with reference to thefollowing embodiments. An ink jet ink (hereinafter referred to simply asan “ink”) according to an embodiment of the present invention contains apolyurethane polymer and a self-dispersing pigment. The polyurethanepolymer has units derived from a polyisocyanate, a polyol having no acidgroup, and at least one selected from dimethylolpropionic acid anddimethylolbutanoic acid. The polyol having no acid group contains apolyether polyol having no acid group, and the percentage (% by mole)constituted by a unit derived from the polyether polyol having no acidgroup with respect to all the units derived from the polyol having noacid group is 80% by mole or more. The polyether polyol has anumber-average molecular weight of 450 or more and 4,000 or less. Theratio of the percentage (% by mole) of urethane bonds in thepolyurethane polymer to the percentage (% by mole) of urea bonds in thepolyurethane polymer is 85.0/15.0 or more and 100/0 or less. The acidvalue of the polyurethane polymer resulting from the unit(s) derivedfrom the at least one selected from dimethylolpropionic acid anddimethylolbutanoic acid is 40 mgKOH/g or more and 140 mgKOH/g or less.

The present inventors studied various pigment inks containing apolyurethane polymer. In accordance with routine procedures, in order toimprove ink ejection stability, polyurethane polymers having a high acidvalue or high hydrophilicity were studied. As a result, it was foundthat some polyurethane polymers provided markedly poor image fastness(scratch resistance and highlighter resistance). As a result ofextensive studies, the present inventors found that the compoundsconstituting the polyurethane polymer are greatly responsible for thepoor image fastness. The details are described below.

The polyurethane polymer is mainly composed of two segments: a hardsegment and a soft segment. The hard segment is mainly composed of apolyisocyanate, a short-chain diol, and a chain extension agent. Thesoft segment is mainly composed of a polyol. The hard segmentsubstantially contributes to high strength, while the soft segmentsubstantially contributes to flexibility. A micro phase separationstructure of the two segments can impart high strength and flexibility,that is, high elasticity to a film of the polyurethane polymer. Suchfilm characteristics are closely related to image fastness.

As described above, a hydrophilic polyurethane polymer is generallyobtained by increasing the acid value of the polyurethane polymer. Theacid value of the polyurethane polymer depends substantially on thenumber of units derived from a compound having an acid group in thepolyurethane polymer. Thus, in order to increase the acid value of thepolyurethane polymer, the amount of compound having an acid group usedin the synthesis of the polyurethane polymer must be increased. Ingeneral, the compound having an acid group for use in the synthesis ofthe polyurethane polymer may be (a) a polyol having an acid group or (b)a diol having an acid group.

The (a) polyol having an acid group may be an acid-modified polyol, suchas a carboxylic-acid-modified polycaprolactone diol. However, it wasfound that the synthesis of the polyurethane polymer only using a polyolhaving an acid group as a compound having an acid group results inmarked decreases in the scratch resistance and highlighter resistance ofan image. This phenomenon occurred not only in the case of high acidvalues but also in the case of low acid values, that is, in the casethat the amount of polyol having an acid group was small. This isprobably because the polyurethane polymer synthesized using a polyolhaving an acid group has the acid group in the soft segment andconsequently has a poor balance between strength and flexibility. Thus,it was found that even when a polyurethane polymer synthesized onlyusing a polyol having an acid group as a compound having an acid groupis used in an ink, high ink ejection stability and image fastness cannotbe achieved.

The (b) diol having an acid group may be dimethylolpropionic acid(hereinafter referred to as DMPA) or dimethylolbutanoic acid(hereinafter referred to as DMBA). An increase in the amount of DMPA orDMBA to increase the acid value of the polyurethane polymer results inan increase in the number of hydroxy groups. Thus, an increase in theamount of DMPA or DMBA used must be compensated for by a relativedecrease in the amount of polyol component, which has a hydroxy groupreactive with an isocyanate like DMPA or DMBA. This decreases the numberof soft segments, reduces the flexibility of the polyurethane polymer,and increases the rigidity of the resulting polyurethane polymer film,thus lowering image fastness. Thus, it was found that use of apolyurethane polymer having a high acid value due to a diol having anacid group, such as DMPA or DMBA, in an ink can improve ink ejectionstability but lowers image fastness.

The present inventors found that a factor in the markedly poor imagefastness by the use of a polyurethane polymer having a high acid valueor high hydrophilicity is greatly related to the structure of thepolyurethane polymer. The present inventors also found that thepolyurethane polymer must be designed particularly in terms of the twosegment structures, the hard segment and the soft segment, which arecharacteristic of the polyurethane polymer, rather than changing thehydrophilicity and hydrophobicity of the polyurethane polymer withrespect to its acid value.

In consideration of these results, the present inventors examinedvarious polyurethane polymers and found that it is important to use atleast one diol having an acid group selected from DMPA and DMBA as acompound having an acid group used in the synthesis of the polyurethanepolymer. It is also important that the acid value of the polyurethanepolymer resulting from the unit(s) derived from the diol having an acidgroup is 40 mgKOH/g or more and 140 mgKOH/g or less and that a polyetherpolyol having no acid group and a number-average molecular weight of 450or more and 4,000 or less is used as a polyol. The reasons for these aredescribed below in detail.

In general, a polyester polyol, a polyether polyol, a polycarbonatediol, or a polycaprolactone polyol is used as a polyol in the synthesisof the polyurethane polymer. Among these, a polyether polyol is veryflexible. Thus, even when a diol having an acid group is used in thesynthesis of the polyurethane polymer, a soft segment composed of apolyether polyol rarely causes a reduction in flexibility due to adecrease in the number of soft segments as described above. Thus, evenwith a somewhat high acid value, the polyurethane polymer can have highflexibility. The present inventors found that when the acid value of thepolyurethane polymer resulting from the unit(s) derived from the diolhaving an acid group is 140 mgKOH/g or less the polyurethane polymer canhave high flexibility and excellent image fastness. On the other hand,when the acid value of the polyurethane polymer resulting from theunit(s) derived from the diol having an acid group is less than 40mgKOH/g, the prerequisite ink ejection stability may be lowered. Thus,when a polyether polyol and a diol having an acid group are used, theacid value of the polyurethane polymer resulting from the unit(s)derived from the diol having an acid group must be 40 mgKOH/g or moreand 140 mgKOH/g or less.

It is also found that the flexibility of the polyether polyol alsogreatly depends on its molecular weight. The present inventors studiedthe molecular weight of the polyether polyol and found that thepolystyrene-equivalent number-average molecular weight of the polyetherpolyol must be 450 or more and 4,000 or less as determined by gelpermeation chromatography (GPC). When the polyether polyol has amolecular weight of less than 450, the polyether polyol has lowflexibility because of its short molecular chain, and the resultingpolyurethane polymer has low flexibility relative to its strength. Onthe other hand, when the polyether polyol has a molecular weight of morethan 4,000, the polyether polyol has excessively high flexibilitybecause of its long molecular chain, and the resulting polyurethanepolymer has low strength relative to its flexibility. In both cases, thepolyurethane polymer has a poor balance between strength andflexibility, resulting in low scratch resistance and highlighterresistance of an image.

The polyurethane polymer described above can be used to significantlyimprove ink ejection stability and image fastness (scratch resistanceand highlighter resistance). However, the present inventors found thatthe ejection of such a polyurethane polymer through an ink jet recordinghead may cause another problem of face wetting, which results in imagedeflection.

The present inventors examined various polyurethane polymers and foundthat a polyurethane polymer having a larger number of urea bonds morefrequently causes face wetting. This is probably because two N—H bondsof one urea bond interact with a face to deposit the polyurethanepolymer on the face.

On the basis of these findings, polyurethane polymers having differentmolar ratios of the urethane bond to the urea bond were examined forimage deflection. As a result, it was found that a polyurethane polymerthat has the structure described above and a smaller number of ureabonds less frequently causes image deflection. More specifically, theratio of the percentage (% by mole) of the urethane bond to thepercentage (% by mole) of the urea bond in the polyurethane polymer mustbe 85.0/15.0 or more and 100.0/0 or less. A method for controlling themolar ratio will be described below.

It was also found that use of a polyurethane polymer that has a smallernumber of urea bonds can further improve ink ejection stability. This isprobably because a decrease in the number of hydrogen bonds formed bythe urea bonds results in weaker intramolecular or intermolecularinteraction of the polymer and an increase in the degree of freedom ofthe polymer molecule in the ink, which facilitates the addition of awater molecule to an acid group of the polymer molecule, therebyincreasing hydrophilicity. Although the urethane bond can also form thehydrogen bond, its hydrogen bond strength is smaller than that of theurea bond. Thus, the urethane bond does not prevent the addition ofwater molecule to an acid group of the polymer molecule.

In addition to the use of such a polyurethane polymer that has a smallernumber of urea bonds, use of a self-dispersing pigment can achieve highoptical density, as well as high scratch resistance and highlighterresistance of an image. The present inventors believe the reason forthat is as follows.

In general, self-dispersing pigments have a higher pigment aggregationspeed associated with water evaporation than polymer-dispersed pigments.Thus, a self-dispersing pigment aggregates rapidly on a recordingmedium. Pigment particles remaining on the surface of the recordingmedium can yield a high optical density.

A urea bond of the polyurethane polymer can easily form a hydrogen bondwith a functional group of a self-dispersing pigment particle. Thepolyurethane polymer interacting with the surface of the self-dispersingpigment particle can function as a polymer dispersant and stabilize thedispersion of the self-dispersing pigment. This prevents rapidaggregation of the self-dispersing pigment on the recording medium.Thus, high optical density intrinsic to the self-dispersing pigmentcannot be achieved. Use of a polyurethane polymer that has a smallernumber of urea bonds can reduce the interaction between the surface of apigment particle and the polyurethane polymer, thus allowing rapidaggregation of the pigment on the recording medium. The self-dispersingpigment can therefore exhibit its intrinsic high optical density.

Thus, these constituents can synergistically produce their effects toachieve the advantages of the present invention.

Ink Jet Ink

The components of an ink jet ink according to an embodiment of thepresent invention will be described below.

Polyurethane Polymer

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention will be described below in detail.

Polyisocyanate

The term “polyisocyanate”, as used herein, refers to a compound havingtwo or more isocyanate groups. Examples of the polyisocyanate for use inan embodiment of the present invention include, but are not limited to,aliphatic polyisocyanates, alicyclic polyisocyanates, aromaticpolyisocyanates, and araliphatic polyisocyanates. The percentage (% bymass) of the unit(s) derived from the polyisocyanate in the polyurethanepolymer may be 10% by mass or more and 80% by mass or less.

Examples of the aliphatic polyisocyanate include, but are not limitedto, tetramethylene diisocyanate, dodecamethylene diisocyanate,hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate,2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate,2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate.Examples of the alicyclic polyisocyanate include, but are not limitedto, isophorone diisocyanate, hydrogenated xylylene diisocyanate,4,4-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate,methylcyclohexylene diisocyanate, and1,3-bis(isocyanatomethyl)cyclohexane. Examples of the aromaticpolyisocyanate include, but are not limited to, tolylene diisocyanate,2,2-diphenylmethane diisocyanate, 2,4-diphenylmethane diisocyanate,4,4-diphenylmethane diisocyanate, 4,4-dibenzyl diisocyanate,1,5-naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylenediisocyanate, and 1,4-phenylene diisocyanate. Examples of thearaliphatic polyisocyanate include, but are not limited to,dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethanediisocyanate, and alpha,alpha,alpha,alpha-tetramethylxylylenediisocyanate. These polyisocyanates may be used alone or in combination.Among these polyisocyanates, hexamethylene diisocyanate may be used inan embodiment of the present invention. Hexamethylene diisocyanate maybe used in combination with another polyisocyanate. The reason for thisis described below.

Hexamethylene diisocyanate (hereinafter referred to as HDI) has astraight chain structure, small steric hindrance, and high molecularsymmetry. In a polyurethane polymer synthesized using HDI, therefore,HDI molecules tend to gather because of hydrogen bonds formed byurethane bonds. Thus, in the polyurethane polymer synthesized using HDI,hard segments containing the polyisocyanate are localized. This tends toresult in micro phase separation in which the hard segments are presentin a continuous soft segment to form a sea-island structure, thusmarkedly increasing the flexibility of the polyurethane polymer. Incontrast, a polyisocyanate that has an intramolecular branched or ringstructure has large steric hindrance and rarely forms a hydrogen bond.However, an interaction between the ring structures and hydrophobicinteraction increase the number of hard segments and markedly increasethe strength of the polyurethane polymer. Thus, use of HDI and anotherpolyisocyanate in combination imparts flexibility due to HDI andstrength due to that other polyisocyanate to the polyurethane polymer,thereby achieving higher scratch resistance and highlighter resistanceof an image.

The percentage (% by mole) constituted by the unit(s) derived from HDIwith respect to all the units derived from the polyisocyanate in thepolyurethane polymer may be 10% by mole or more and 90% by mole or less.The balance of the effect of improving strength and flexibility betweenHDI and the polyisocyanate other than HDI is satisfactory within thisrange. This further improves the scratch resistance and highlighterresistance of an image.

Polyether Polyol Having No Acid Group

As described above, the polyether polyol having no acid group for use inan embodiment of the present invention has a polystyrene-equivalentnumber-average molecular weight of 450 or more and 4,000 or less asdetermined by GPC. The percentage (% by mass) of the unit(s) derivedfrom the polyether polyol having no acid group in the polyurethanepolymer may be 0.1% by mass or more and 80.0% by mass or less. In oneembodiment of the present invention, the polyether polyol having no acidgroup may be used in combination with another polyol having no acidgroup other than the polyether polyol to synthesize the polyurethanepolymer. In this case, the percentage (% by mole) constituted by theunit(s) derived from the polyether polyol having no acid group withrespect to all the units derived from the polyol having no acid group inthe polyurethane polymer may be 80% by mole or more and 100% by mole orless. In particular, no castor-oil-modified polyol may be used in viewof ink ejection stability.

Examples of the polyether polyol include, but are not limited to,poly(alkylene glycol)s and addition polymers of alkylene oxides anddihydric alcohols or at least trivalent polyhydric alcohols. Examples ofthe poly(alkylene glycol)s include, but are not limited to,poly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol), poly(1,2-butylene glycol), poly(1,3-butylene glycol), andethylene glycol-propylene glycol copolymers. Examples of the dihydricalcohols include, but are not limited to, hexamethylene glycol,tetramethylene glycol, ethylene glycol, diethylene glycol, propyleneglycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol,4,4-dihydroxyphenylpropane, and 4,4-dihydroxyphenylmethane. Examples ofthe at least trivalent polyhydric alcohols include, but are not limitedto, glycerin, trimethylolpropane, 1,2,5-hexanetriol, 1,2,6-hexanetriol,and pentaerythritol. Examples of the alkylene oxides include, but arenot limited to, ethylene oxide, propylene oxide, butylene oxide, andalpha-olefin oxides. These polyether polyols may be used alone or incombination.

In one embodiment of the present invention, the polyether polyol havingno acid group may contain at least one selected from poly(ethyleneglycol), poly(propylene glycol), poly(1,2-butylene glycol), andpoly(1,3-butylene glycol). Use of these polyether polyols can increasethe strength, flexibility, and hydrophilicity of the polyurethanepolymer, thereby further improving the scratch resistance andhighlighter resistance of an image and ink ejection stability. Thepercentage (% by mole) constituted by the unit(s) derived frompoly(ethylene glycol), poly(propylene glycol), poly(1,2-butyleneglycol), and poly(1,3-butylene glycol) with respect to all the unitsderived from the polyether polyol in the polyurethane polymer may be 80%by mole or more and 100% by mole or less. In particular, the polyetherpolyol having no acid group may contain poly(propylene glycol). Use ofpoly(propylene glycol) can improve the balance between the strength andthe flexibility of the polyurethane polymer film.

Diol Having Acid Group

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention has a unit derived from at least one selected fromDMPA and DMBA as a diol having an acid group. The diol having an acidgroup may be in the form of a salt with an alkali metal, such as Li, Na,or K, or an organic amine, such as ammonia or dimethylamine. These diolsmay be used alone or in combination. The percentage (% by mass) of theunit(s) derived from the diol having an acid group in the polyurethanepolymer may be 5.0% by mass or more and 40.0% by mass or less.

Chain Extension Agent

A chain extension agent is a compound that can react with a residualisocyanate group of a polyisocyanate unit of a urethane prepolymer. Theresidual isocyanate group is an isocyanate group that did not form aurethane bond. In one embodiment of the present invention, a chainextension agent may be used in the synthesis of the polyurethane polymerprovided that the molar ratio of the urethane bond to the urea bond inthe polyurethane polymer is 85.0/15.0 or more and 100.0/0 or less.Examples of the chain extension agent include, but are not limited to,polyvalent amine compounds, such as trimethylolmelamine and derivativesthereof, dimethylolurea and derivatives thereof, dimethylolethylamine,diethanolmethylamine, dipropanolethylamine, dibutanolmethylamine,ethylenediamine, propylenediamine, diethylenetriamine, hexylenediamine,triethylenetetramine, tetraethylenepentamine, isophoronediamine,xylylenediamine, diphenylmethanediamine, hydrogenateddiphenylmethanediamine, and hydrazine, polyamide polyamine, andpolyethylene polyimine. Examples of the chain extension agent alsoinclude, but are not limited to, ethylene glycol, propylene glycol,1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, dipropylene glycol, tripropylene glycol,poly(ethylene glycol), 3-methyl-1,5-pentanediol,2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanediol,1,4-cyclohexanedimethanol, hydrogenated bisphenol A, glycerin,trimethylolpropane, and pentaerythritol. These chain extension agentsmay be used alone or in combination.

Cross-linker

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention may be a cross-linked polyurethane polymer. Thecross-linked polyurethane polymer may be produced by using an at leasttrifunctional polyisocyanate, polyol, or chain extension agent, whichfunctions as a cross-linker.

The cross-linked polyurethane polymer is rich in the hard segment of themicro phase separation structure of the polyurethane polymer. This isbecause use of, for example, a trifunctional cross-linker results in theformation of three urethane bonds per cross-linker. This markedlyincreases the strength of the polyurethane polymer. This also increasesthe number of adjacent urethane bonds and accordingly the number ofhydrogen bonds between the urethane bonds. Consequently, the hardsegments gather more closely and easily undergo micro phase separationto form a sea-island structure, maintaining high flexibility of thepolyurethane polymer. Thus, the cross-linked polyurethane polymer hasvery high strength and high flexibility and improves the scratchresistance and highlighter resistance of an image.

Examples of at least trifunctional compound that can be used as across-linker in an embodiment of the present invention include, but arenot limited to, at least trifunctional polyisocyanates, at leasttrifunctional polyols, and at least trifunctional chain extensionagents. The cross-linked polyurethane polymer can be synthesized usingat least one selected from the at least trifunctional compounds.Examples of the at least trifunctional polyisocyanates include, but arenot limited to, polyisocyanurates, adduct-type polyisocyanates, andbiuret polyisocyanates. Examples of the at least trifunctional polyolsinclude, but are not limited to, glycerin, trimethylolpropane,1,2,5-hexanetriol, 1,2,6-hexanetriol, pentaerythritol,polyoxypropylenetriol, and glycol adducts of the polyether polyolsdescribed above. Examples of the at least trifunctional chain extensionagent include, but are not limited to, trimethylolmelamine,diethylenetriamine, triethylenetetramine, and tetraethylenepentamine. Inparticular, at least one selected from glycerin, trimethylolpropane,pentaerythritol, and polyoxypropylenetriol may be used.

Method for Determining Whether Cross-linked Polyurethane Polymer or Not

The present inventors found that whether the polyurethane polymer is across-linked polyurethane polymer or not can be determined by gelfraction. When a polyurethane polymer is dissolved in a solvent, the gelfraction of the polyurethane polymer is the ratio of the mass ofresidual gel (a cross-linked structure remains as gel) to the mass ofthe polyurethane polymer before dissolved in the solvent. The gelfraction can be measured by the solubility of a polyurethane polymerfilm in a solvent and is a measure of the degree of cross-linking. Apolyurethane polymer having a higher degree of cross-linking has ahigher gel fraction. In one embodiment of the present invention, the gelfraction is measured by the following method to determine whether thepolyurethane polymer is a cross-linked polyurethane polymer or not.

A film of a polyurethane polymer extracted by a method described belowis immersed in tetrahydrofuran (THF) at 23 degrees Celsius for 24 hours.The mass (A) of a THF-insoluble fraction of the polyurethane polymerfilm is measured and is divided by the mass (B) of the polyurethanepolymer film before immersion to calculate a THF gel fraction (A/B×100).In one embodiment of the present invention, a polyurethane polymerhaving a THF gel fraction of 88% by mass or more and 100% by mass orless is considered to be a cross-linked polyurethane polymer.

Molar Ratio of Urethane Bond to Urea Bond

As described above, the ratio of the percentage (% by mole) of theurethane bond to the percentage (% by mole) of the urea bond in thepolyurethane polymer for use in an ink according to an embodiment of thepresent invention is 85.0/15.0 or more and 100.0/0 or less. In oneembodiment of the present invention, the molar ratio of the urethanebond to the urea bond may be 85.0/15.0 or more and 98.5/1.5 or less.When the molar ratio of the urethane bond to the urea bond is 98.5/1.5or less, the scratch resistance and highlighter resistance of an imageare further improved.

In one embodiment of the present invention, the molar ratio of theurethane bond to the urea bond of the polyurethane polymer may becontrolled by either of the following two methods.

In accordance with a first method, the amount of amine compound used inthe synthesis of the polyurethane polymer is controlled. This is becausethe amine compound reacts with an isocyanate group to form a urea bond.The amine compound may be an amine chain extension agent. Morespecifically, the polyurethane polymer may be synthesized by thefollowing method. First, polyurethane polymers are synthesized usingdifferent amounts of an amine compound. The molar ratio of the urethanebond to the urea bond of each of the polyurethane polymers is measuredby the method described below. On the basis of these results, thecorrelation between the amount of amine compound used and the molarratio of the urethane bond to the urea bond is examined to determine theamounts of raw materials for a polyurethane polymer having a desiredmolar ratio of the urethane bond to the urea bond. The reason that thecorrelation between the amount of amine compound used and the molarratio is examined in advance is as follows. Even with the same aminecompound, use of different types of other raw materials may result in achange in the reaction rate or the like, yielding polyurethane polymershaving different molar ratios of the urethane bond to the urea bond.

In accordance with a second method, the percentage of the unreactedisocyanate group is controlled in phase inversion of the polyurethanepolymer to water. Water reacts with an isocyanate group to form a ureabond. More specifically, the polyurethane polymer is synthesized by thefollowing method. First, the percentage of the unreacted isocyanategroup in the synthesis of the polyurethane polymer is measured with aFourier transform infrared spectrophotometer (FT-IR). When thepercentage of the unreacted isocyanate group reaches the desired molarratio of the urethane bond to the urea bond, ion-exchanged water isadded to the reaction system. For example, in the synthesis of apolyurethane polymer having a molar ratio of the urethane bond to theurea bond of 95.0/5.0, ion-exchanged water is added when the percentageof the unreacted isocyanate group is 5% by mole. The percentage of theunreacted isocyanate group may be controlled by the reaction time or theinitial amount of polyisocyanate. In the examples described below, themolar ratio of the urethane bond to the urea bond of the polyurethanepolymer was controlled by the second method.

Characteristics of Polyurethane Polymer

The percentage (% by mass) of the polyurethane polymer for use in an inkaccording to an embodiment of the present invention may be 0.1% by massor more and 10.0% by mass or less of the total mass of the ink Less than0.1% by mass of the polyurethane polymer may have an insufficient effectof improving the scratch resistance and highlighter resistance of animage. More than 10.0% by mass of the polyurethane polymer may have aninsufficient effect of improving ink ejection stability. The ink mayfurther contain another polymer without compromising the advantages ofthe present invention.

The ratio of the polyurethane polymer content (% by mass) of the ink tothe self-dispersing pigment content (% by mass) based on the total massof the ink described below may be 0.05 or more and 2.00 or less. A massratio of less than 0.05 may result in an insufficient effect ofimproving the scratch resistance and highlighter resistance of an image.When the mass ratio is more than 2.00, this may result in aninsufficient effect of improving ink ejection stability.

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention may have a polystyrene-equivalent weight-averagemolecular weight (Mw) of more than 30,000 and 150,000 or less asdetermined by GPC. When the polystyrene-equivalent weight-averagemolecular weight (Mw) is 30,000 or less, the polyurethane polymer mayhave low strength and an insufficient effect of improving the scratchresistance and highlighter resistance of an image. When thepolystyrene-equivalent weight-average molecular weight (Mw) is more than150,000, the ink tends to have a high viscosity, and this may result inan insufficient effect of improving ink ejection stability.

A polyurethane polymer for use in an ink according to an embodiment ofthe present invention may have an acid value of 40 mgKOH/g or more and150 mgKOH/g or less. As described above, the acid value of thepolyurethane polymer resulting from the unit(s) derived from the diolhaving an acid group must be 40 mgKOH/g or more and 140 mgKOH/g or less.The acid value of the polyurethane polymer resulting from the unit(s)derived from the diol having an acid group may be 45 mgKOH/g or more and100 mgKOH/g or less.

Method for Synthesizing Polyurethane Polymer

A method for synthesizing a polyurethane polymer according to anembodiment of the present invention may be any known method, includingthe following method. A polyisocyanate, a polyol having no acid group,and a diol having an acid group are allowed to react to synthesize aurethane prepolymer having a terminal isocyanate group. An acid group ofthe urethane prepolymer is then neutralized with a neutralizing agent.The neutralized urethane prepolymer is further allowed to react in anaqueous solution containing a chain extension agent or a cross-linker.An organic solvent remaining in the system, if any, may be removed.

The present inventors found that, when two polyisocyanates (a firstpolyisocyanate and a second polyisocyanate) are used to synthesize apolyurethane polymer having a decreased number of urea bonds, ahigh-molecular-weight polyurethane polymer can be produced by thefollowing method while the molar ratio of the urethane bond to the ureabond is controlled. More specifically, the first polyisocyanate, apolyol having no acid group, and a diol having an acid group are allowedto react. The reaction is continued until the percentage of theunreacted isocyanate group in the reaction system becomes zero asmeasured with an FT-IR, yielding a prepolymer solution. This prepolymercontains no isocyanate group of the first polyisocyanate. The secondpolyisocyanate is added to the prepolymer solution and is allowed toreact until the percentage of the unreacted isocyanate group in thereaction system reaches a desired value as measured with an FT-IR. Achain extension agent or a cross-linker is then added to the prepolymersolution and is allowed to react to yield a polyurethane polymersolution. The phase inversion of the polyurethane polymer solution towater yields a polyurethane polymer dispersion having a desired molarratio of the urethane bond to the urea bond. In this synthesis method,when the first polyisocyanate is isophorone diisocyanate, the scratchresistance and highlighter resistance of an image are improved. Whethera polyurethane polymer is synthesized by this synthesis method or notmay be determined by the following method. When the molar ratio X of theurethane bond formed between the first polyisocyanate and the diolhaving an acid group to the first polyisocyanate is larger than themolar ratio Y of the urethane bond formed between the secondpolyisocyanate and the diol having an acid group to the secondpolyisocyanate, the polyurethane polymer is considered to be synthesizedby the synthesis method described above. X and Y can be determined bynuclear magnetic resonance (NMR) as described below.

Method for Analyzing Polyurethane Polymer

The composition, the molecular weight, and the acid value of theresulting polyurethane polymer can be analyzed by examining the sedimentand the supernatant liquid after the centrifugation of the ink Since thepigment is insoluble in organic solvents, the polyurethane polymer canalso be isolated by solvent extraction. Although the ink itself can beanalyzed, the isolation of the polyurethane polymer can improve theaccuracy of measurement. More specifically, after the centrifugation ofthe ink at 80,000 rpm, the supernatant liquid is precipitated withhydrochloric acid and is dried.

(1) Polyurethane Polymer Composition

The dried precipitate is dissolved in deuterated dimethyl sulfoxide(deuterated DMSO) and is subjected to proton nuclear magnetic resonance(¹H-NMR). The types of the polyisocyanate, the polyether polyol havingno acid group, and the diol having an acid group can be determined bythe peak positions of the proton nuclear magnetic resonance and themeasurement of the dried precipitate by pyrolysis-gas chromatography.The component ratio can be calculated from the ratio of the integralvalues of chemical shift peaks. The percentage constituted by theunit(s) derived from HDI with respect to all the units derived from thepolyisocyanate in the polyurethane polymer can be calculated by thismethod.

(2) Method for Measuring Molar Ratio of Urethane Bond to Urea Bond ofPolyurethane Polymer

The molar ratio of the urethane bond to the urea bond of thepolyurethane polymer can be determined by the ratio of the integralvalue of the peaks of the urethane bond to the integral value of thepeaks of the urea bond of the polyurethane polymer precipitate dissolvedin deuterated DMSO measured by carbon nuclear magnetic resonance(¹³C-NMR). The peak positions of the urethane bond and the urea bonddepend on the types of raw material compounds of the polyurethanepolymer (a polyisocyanate, a polyether polyol having no acid group, anda diol having an acid group). Thus, the peak positions of the urethanebond and the urea bond for the raw material compounds of thepolyurethane polymer must be examined by the following method.

First, the raw material compounds of the polyurethane polymer (apolyisocyanate, a polyether polyol having no acid group, and a diolhaving an acid group) are prepared. (i) A reaction product between thepolyisocyanate and the polyether polyol having no acid group, (ii) areaction product between the polyisocyanate and the diol having an acidgroup, and (iii) a reaction product between the polyisocyanate and waterare obtained. Each of the products is then dried, is dissolved indeuterated DMSO, and is subjected to ¹³C-NMR measurement. The peakposition of the urethane bond in each of the reaction products can beidentified from the results for (i) and (ii). The peak position of theurea bond in each of the reaction products can be identified from theresult for (iii). For example, when the polyisocyanate is isophoronediisocyanate, the peak position of the urethane bond is approximately155 ppm, and the peak position of the urea bond is approximately 158ppm.

(3) Method for Measuring Acid Value of Polyurethane Polymer

The acid value of the polyurethane polymer can be measured bytitrimetry. In the examples described below, the acid value of thepolyurethane polymer was measured by potentiometric titration of thepolymer dissolved in THF with an automatic potentiometric titrator AT510(manufactured by Kyoto Electronics Manufacturing Co., Ltd.) using apotassium hydroxide-ethanol titrant.

The acid value of the diol having an acid group (DMPA or DMBA) can bemeasured by the following method. First, a polyurethane polymerprecipitate dissolved in deuterated DMSO is subjected to ¹³C-NMRmeasurement. The molar ratio of DMPA or DMBA to another compound havingan acid group (for example, acrylic acid) is calculated by the ratio ofthe peak of the quaternary carbon atom of DMPA or DMBA to the integralvalue of the peak of an acid group of that other compound. The acidvalue of the polyurethane polymer is multiplied by the molar ratio ofDMPA or DMBA to calculate the acid value of the diol having an acidgroup, DMPA or DMBA. The peak position of the quaternary carbon atom ofDMPA or DMBA is approximately 65 or 60 ppm, respectively. When nocompound having an acid group other than DMPA or DMBA is used, the acidvalue of the diol having an acid group can be measured by thetitrimetry.

(4) Method for Measuring Average Molecular Weight of PolyurethanePolymer

The average molecular weight of the polyurethane polymer can be measuredby GPC. In GPC measurement, an apparatus Alliance GPC 2695 (manufacturedby Waters), four columns of Shodex KF-806M (manufactured by Showa DenkoK.K.) in series, and a RI (refractive index) detector were used. Theaverage molecular weight was calculated using polystyrene standardsamples PS-1 and PS-2 (manufactured by Polymer Laboratories).

(5) Method for Measuring Number-average Molecular Weight of PolyetherPolyol Having No Acid Group

The polyurethane polymer dissolved in deuterated DMSO can be subjectedto carbon nuclear magnetic resonance (¹³C-NMR) to calculate the numberof polyether polyol units having no acid group and the number-averagemolecular weight of the polyether polyol.

Self-dispersing Pigment

An ink according to an embodiment of the present invention contains aself-dispersing pigment in which at least one hydrophilic group isbonded to the surface of the pigment particle directly or throughanother atomic group. The at least one hydrophilic group in the ink maybe partly or entirely dissociated.

Examples of the pigment for use in an ink according to an embodiment ofthe present invention include, but are not limited to, inorganic andorganic pigments, such as carbon black. Any known pigment can be used inan ink jet ink according to an embodiment of the present invention. Thepresent inventors found that the polyurethane polymer is physicallyadsorbed on inorganic pigments more easily than organic pigments. Use ofinorganic pigments therefore more effectively improves the scratchresistance and highlighter resistance of an image. The self-dispersingpigment content (% by mass) of the ink is 0.1% by mass or more and 15.0%by mass or less, preferably 1.0% by mass or more and 8.0% by mass orless, of the total mass of the ink. A self-dispersing pigment content ofless than 1.0% by mass may result in insufficient optical density. Aself-dispersing pigment content of more than 8.0% by mass may result inpoor ink jet characteristics, such as sticking resistance.

Examples of the self-dispersing pigment include, but are not limited to,pigments having a hydrophilic group on the surface of the pigmentparticle (self-dispersing pigments modified with hydrophilic groups) andpigments having a polymer organic group on the surface of the pigmentparticle (polymer-attached self-dispersing pigments). Use of theself-dispersing pigment can obviate the necessity for the addition of adispersant to disperse the pigment in the ink or can reduce the amountof dispersant. In one embodiment of the present invention, theself-dispersing pigment modified with hydrophilic groups can be used.The pigment can be self-dispersing carbon black. Self-dispersing carbonblack can promote a change in the state of the ink (an increase inviscosity, the association or aggregation of a coloring material, or thelike) as the ink dries on a recording medium. Thus, self-dispersingcarbon black is very effective in achieving high optical density.

Self-dispersing Pigments Modified with Hydrophilic Groups

The hydrophilic group on the surface of the pigment particle may be ananionic group, such as —COOM, —SO₃M, —PO₃HM, or —PO₃M₂, wherein “M”denotes a hydrogen atom, an alkali metal, ammonium, or an organicammonium. When “M” is an alkali metal, such as lithium, sodium, orpotassium, the ink has excellent ejection stability. The hydrophilicgroup may be bonded to the surface of the pigment particle directly orvia another atomic group (—R—). The atomic group (—R—) may be a linearor branched alkylene group having 1 to 12 carbon atoms, an arylenegroup, such as a phenylene group or a naphthylene group, an amide group,a sulfonyl group, an amino group, a carbonyl group, an ester group, oran ether group, or a combination thereof.

Polymer-attached Self-dispersing Pigment

A polymer used in a polymer-attached self-dispersing pigment may be anyknown polymer used in ink jet inks, for example, an acrylic acidpolymer. A polymer attached to the pigment preferably has apolystyrene-equivalent weight-average molecular weight in the range of1,000 to 12,000, more preferably 3,000 to 8,000, as determined by GPC.The polymer preferably has an acid value of 50 mgKOH/g or more and 300mgKOH/g or less, more preferably 120 mgKOH/g or more and 250 mgKOH/g orless.

Aqueous Medium

An ink according to an embodiment of the present invention may containwater or an aqueous medium, such as a mixed solvent of water and awater-soluble organic solvent. The amount (% by mass) of water-solubleorganic solvent in the ink may be 3.0% by mass or more and 50.0% by massor less of the total mass of the ink. The water-soluble organic solventmay be any known solvent generally used in ink jet inks. Examples of thewater-soluble organic solvent include, but are not limited to, alkylalcohols having 1 to 4 carbon atoms, amides, ketones, keto-alcohols,ethers, poly(alkylene glycol)s, glycols, alkylene glycols in which thealkylene group has 2 to 6 carbon atoms, polyhydric alcohols, alkyl etheracetates, alkyl ethers of polyhydric alcohols, nitrogen-containingcompounds, and sulfur-containing compounds. These water-soluble organicsolvents may be used alone or in combination. Water can be deionizedwater (ion-exchanged water). The water content (% by mass) of the inkmay be 50.0% by mass or more and 95.0% by mass or less of the total massof the ink. The ink viscosity at 25 degrees Celsius may be 6 cps orless. The ink viscosity can be controlled through the composition andthe amount of the aqueous medium. An ink viscosity of more than 6 cps at25 degrees Celsius may result in an insufficient effect of improving inkejection stability.

Salt

An ink according to an embodiment of the present invention can alsocontain a salt. The term “salt”, as used herein, refers to a compoundformed by neutralization between a cation and an anion. The salt in theink may be partly or entirely dissociated. The expression “contain asalt” includes the presence of a dissociated salt. The addition of asalt to an ink according to an embodiment of the present invention canpromote the agglomeration of pigment on a recording medium andsignificantly improve optical density.

Examples of the salt for use in an ink according to an embodiment of thepresent invention include, but are not limited to, combinations of thefollowing cations and anions. Specific examples of the cations include,but are not limited to, monovalent metal ions, such as Li⁺, Na⁺, and K⁺,ammonium ion, and organic ammonium ions. Examples of the anions include,but are not limited to, Cl⁻, Br⁻, I⁻, ClO⁻, ClO²⁻, ClO³⁻, ClO⁴⁻, NO²⁻,NO³⁻, SO₄ ²⁻, CO₃ ²⁻, HCOO⁻, CH₃COO⁻, C₂H₄(COO⁻)₂, C₆H₅COO⁻, andC₆H₄(COO⁻)₂.

The salt content (% by mass) of an ink according to an embodiment of thepresent invention may be 0.01% by mass or more and 10.0% by mass or lessof the total mass of the ink. A salt content of less than 0.01% by massmay result in an insufficient effect of improving optical density. Asalt content of more than 10.0% by mass may result in an insufficienteffect of improving ink storage stability.

Other Additive Agents

In addition to the components described above, an ink according to anembodiment of the present invention may further contain a water-solubleorganic compound that is solid at normal temperature, for example, apolyhydric alcohol, such as trimethylolpropane or trimethylolethane,urea, or a urea derivative, such as ethylene urea. An ink according toan embodiment of the present invention may also contain an additiveagent, such as a polymer other than the polyurethane polymer, asurfactant, a pH-adjusting agent, an anticorrosive, a preservative, afungicide, an antioxidant, a reduction inhibitor, an evaporationaccelerator, and/or a chelator. In the case that the ink contains apolymer other than the polyurethane polymer, the total polymer contentof the ink may be 0.01% by mass or more and 10.00% by mass or less ofthe total mass of the ink.

Ink Cartridge

An ink cartridge according to an embodiment of the present inventionincludes an ink storage portion for storing an ink according to anembodiment of the present invention. The ink storage portion may includean ink chamber and a chamber for housing a negative-pressure-generatingmember. The ink chamber can store liquid ink. Thenegative-pressure-generating member can store ink by the action of anegative pressure. Alternatively, an ink cartridge according to anembodiment of the present invention may include no ink chamber andinclude an ink storage portion that includes anegative-pressure-generating member for storing the whole ink.Alternatively, an ink cartridge according to an embodiment of thepresent invention may include an ink storage portion and a recordinghead.

Ink Jet Recording Method

An ink jet recording method according to an embodiment of the presentinvention involves ejecting an ink according to an embodiment of thepresent invention from an ejection port of a recording head onto arecording medium by an ink jet method in response to recording signals.The ink may be ejected from an ejection port of a recording head by theaction of thermal energy. In one embodiment of the present invention,the face of the recording head may be subjected to water-repellentfinishing. The face of the recording head may be subjected towater-repellent finishing by any method. For example, the face of therecording head may be treated with a water repellent made of a siliconematerial or a fluorinated material. Examples of the water repellentinclude, but are not limited to, KP-801 (manufactured by Shin-EtsuChemical Co., Ltd.), Defensa (manufactured by DIC Corp.), CytopCTX-105,805 (manufactured by Asahi Glass Co., Ltd.), and Teflon(registered trademark) AF (manufactured by DuPont). Afluorine-containing silane compound may also be used as a waterrepellent. The term “recording”, as used herein, includes recording on arecording medium with an ink according to an embodiment of the presentinvention or printing on a substrate having little permeability, such asa glass substrate, a plastic substrate, or a non-permeable film, with anink according to an embodiment of the present invention. The recordingmedium may be plain paper or glossy paper. Glossy paper includes aporous ink-absorbing layer on a permeable support (such as paper). Theporous ink-absorbing layer contains an inorganic pigment and a binder.Use of an ink according to an embodiment of the present invention onplain paper can particularly have the advantages of the presentinvention.

An ink jet recording method according to an embodiment of the presentinvention may include a process (A) of ejecting an ink through an inkjet recording head onto a recording medium and a process (B) of applyinga liquid composition described below to the recording medium such thatthe liquid composition at least partly overlaps the ink. The process (A)may be followed or preceded by the process (B). The same processes maybe performed twice or more; for example, the process (A), the process(B), and then the process (A), or the process (B), the process (A), andthen the process (B) may be performed. In particular, performing theprocess (A) after the process (B) has a large effect of improving thescratch resistance and optical density of an image.

Liquid Composition

An ink according to an embodiment of the present invention may be usedin combination with a liquid composition that can destabilize thedispersion of pigment in the ink and reduce the solubility of thepolyurethane polymer. The pigment reacts with a reactant in the liquidcomposition to aggregate rapidly and remain on the surface of arecording medium, thus increasing optical density. The polyurethanepolymer also reacts with a reactant in the liquid composition. Thisreduces the solubility of the polyurethane polymer and causesprecipitation of the polyurethane polymer. Thus, the polyurethanepolymer is present in the vicinity of the aggregated pigment andimproves the scratch resistance and highlighter resistance of an image.The phrase “destabilize the dispersion of pigment in the ink”, as usedherein, means that pigment particles aggregate as a result of areduction in electrostatic repulsion by which the pigment particles aredispersed in the ink The phrase “reduce the solubility of thepolyurethane polymer”, as used herein, means that the polyurethanepolymer, which has been dissolved (dispersed) in the ink by the additionof a water molecule to an acid group of the polyurethane polymer(hydration), is insolubilized by the prevention of hydration.

In one embodiment of the present invention, the liquid composition maybe colorless, milk white, or white so as not to affect an image recordedby the ink. Thus, the ratio Amax/Amin of the maximum absorbance Amax tothe minimum absorbance Amin in a visible light wavelength in the rangeof 400 to 780 nm may be 1.0 or more and 2.0 or less. This means that theabsorption peak is substantially absent or very small in the visiblelight wavelength region. A liquid composition according to an embodimentof the present invention may contain no coloring material. Theabsorbance may be measured after the liquid composition is diluted. Thisis because the maximum absorbance Amax and the minimum absorbance Aminof the liquid composition are proportional to the dilution ratio, andtherefore the Amax/Amin is independent of the dilution ratio. Thecomponents of the liquid composition are described below.

Reactant

In one embodiment of the present invention, the liquid composition maycontain a reactant that can destabilize the dispersion of pigment andreduce the solubility of the polyurethane polymer in the ink. Morespecifically, the reactant may be a polyvalent metal ion or an organicacid. The reactant content (% by mass) of the liquid composition may be3.0% by mass or more and 20.0% by mass or less of the total mass of theliquid composition. Less than 3.0% by mass of the reactant cannotsufficiently destabilize the pigment dispersion and may have aninsufficient effect of improving optical density. More than 20.0% bymass of the reactant may cause the precipitation of the reactant andresult in poor ink jet characteristics.

Polyvalent Metal Ion

In one embodiment of the present invention, the polyvalent metal ion ofthe liquid composition may be at least divalent. Examples of thedivalent metal ion include, but are not limited to, alkaline-earthmetals, such as beryllium, magnesium, calcium, strontium, barium, andradium. Examples of at least trivalent metal ion include, but are notlimited to, aluminum, yttrium, zirconium, iron, and other transitionmetal ions. In one embodiment of the present invention, the polyvalentmetal ion may be added in the form of salt, such as hydroxide, chloride,or nitrate, to the liquid composition. The polyvalent metal ion may alsobe a dissociated ion.

In one embodiment of the present invention, at least one selected fromcalcium ion, aluminum ion, and yttrium ion may be used in terms ofreactivity. In particular, calcium ion can be used. A nitrate may beused in terms of the solubility of the salt. One example of nitrate iscalcium nitrate.

Organic Acid

The term “organic acid”, as used herein, refers to an acid of an organiccompound. In one embodiment of the present invention, the organic acidof the liquid composition may be a monocarboxylic acid, such as formicacid, acetic acid, propionic acid, or butyric acid; a dicarboxylic acid,such as oxalic acid, malonic acid, succinic acid, or glutaric acid; or ahydroxycarboxylic acid, such as malic acid or tartaric acid. In oneembodiment of the present invention, the organic acid may be added inthe form of alkali metal ion salt to the liquid composition or may alsobe a dissociated ion.

When the reactant is an organic acid, the liquid composition may have apH of 3.5 or more and 5.5 or less. When the liquid composition has a pHof less than 3.5, this may result in the acid corrosion of a member ofan ink jet recording apparatus. When the liquid composition has a pH ofmore than 5.5, this may result in an insufficient effect of improvingthe scratch resistance and highlighter resistance of an image. The pH ofthe liquid composition is measured at 25 degrees Celsius and can bemeasured with a common pH meter. When the reactant is an organic acid,the liquid composition may have pH-buffering action. The sentence “theliquid composition has pH-buffering action”, as used herein, means thatan equivalent mixture of the liquid composition and the ink hassubstantially the same pH as the liquid composition. The phrase“substantially the same pH as the liquid composition”, as used herein,refers to a pH change of less than 0.1.

Aqueous Medium and Another Additive Agent

The liquid composition may contain water or an aqueous medium, such as amixed solvent of water and a water-soluble organic solvent. The amount(% by mass) of water-soluble organic solvent in the liquid compositionmay be 3.0% by mass or more and 50.0% by mass or less of the total massof the liquid composition. The water-soluble organic solvent may be thewater-soluble organic solvent that can be used in the ink describedabove. Water can be deionized water (ion-exchanged water). The watercontent (% by mass) of the liquid composition may be 50.0% by mass ormore and 95.0% by mass or less of the total mass of the liquidcomposition. The liquid composition may contain the additive agent(s)described above for the ink. In particular, in one embodiment of thepresent invention, the pH of the liquid composition may be adjusted to3.5 or more and 5.5 or less with a pH-adjusting agent. Examples of thepH-adjusting agent include, but are not limited to, organic acids, suchas acetic acid and methanesulfonic acid, inorganic acids, such assulfuric acid and nitric acid, and bases, such as alkali metalhydroxide.

EXAMPLES

The present invention will be further described in the followingexamples and comparative examples. However, the present invention is notlimited to these examples. Unless otherwise specified, “part” in theexamples is based on mass. The following are abbreviations.

-   IPDI: isophorone diisocyanate-   MDI: dicyclohexylmethane diisocyanate-   HDI: hexamethylene diisocyanate-   PPG: poly(propylene glycol)-   PEG: poly(ethylene glycol)-   P(12BG): poly(1,2-butylene glycol)-   P(13BG): poly(1,3-butylene glycol)-   PTMG: poly(tetramethylene glycol)-   PC: polycarbonate diol-   PES: polyester polyol-   PCL: polycaprolactone polyol-   DMPA: dimethylolpropionic acid-   DMBA: dimethylolbutanoic acid-   TMP: trimethylolpropane-   Gly: glycerin-   PE: pentaerythritol-   PPT: polyoxypropylenetriol (Actcol 32-160, manufactured by Takeda    Pharmaceutical Co., Ltd.)-   EDA: ethylenediamine    Preparation of Polyurethane Polymer Dispersion    Preparation of Polyurethane Polymer Dispersions PU-1 to PU-40

A polyisocyanate (A part and B part), a polyol (C part), a diol havingan acid group (D part), and methyl ethyl ketone (300 parts) in afour-neck flask equipped with a thermometer, an agitator, a nitrogeninlet, and a reflux condenser were allowed to react in a nitrogen gasatmosphere at 80 degrees Celsius for six hours. A cross-linker (E part)was added to the flask and was allowed to react at 80 degrees Celsiusuntil a desired molar ratio of the urethane bond to the urea bond wasobtained while the percentage of the unreacted isocyanate group wasmeasured with an FT-IR. The molar ratio of the urethane bond to the ureabond was controlled by the method described above in detail. Aftercompleting the reaction and cooling to 40 degrees Celsius, ion-exchangedwater was added to the flask, and aqueous potassium hydroxide was addedto the flask while stirring with a homomixer at a high speed. Thepolymer solution was heated under reduced pressure to evaporate methylethyl ketone, yielding polyurethane polymer dispersions PU-1 to PU-40each having a solid content of 20% by mass and a weight-averagemolecular weight of 35,000 or more. Table 1 shows the preparationconditions for the polyurethane polymer dispersions. The acid value, themolar ratio of the urethane bond to the urea bond, and the gel fractionof each of the polyurethane polymers were measured by the methoddescribed above. Table 2 shows the properties of the polyurethanepolymer dispersions.

Preparation of Polyurethane Polymer Dispersion PU-45

IPDI (49.5 parts), PPG (number-average molecular weight: 2,000) (103.7parts), DMPA (28.7 parts), and methyl ethyl ketone (300 parts) in afour-neck flask equipped with a thermometer, an agitator, a nitrogeninlet, and a reflux condenser were allowed to react in a nitrogen gasatmosphere at 80 degrees Celsius for 7 hours. The percentage of theunreacted isocyanate group was zero as measured with an FT-IR. HDI (14.3parts) was added to the flask and was allowed to react at 80 degreesCelsius. A cross-linker TMP (3.8 parts) was added to the flask and wasallowed to react at 80 degrees Celsius until the molar ratio of theurethane bond to the urea bond reached 95.0/5.0 while the percentage ofthe unreacted isocyanate group was measured with an FT-IR. Aftercompleting the reaction and cooling to 40 degrees Celsius, ion-exchangedwater was added to the flask, and aqueous potassium hydroxide was addedto the flask while stirring with a homomixer at a high speed. Thepolymer solution was heated under reduced pressure to evaporate methylethyl ketone, yielding a polyurethane polymer dispersion PU-45 having asolid content of 20% by mass and a weight-average molecular weight of50,000. The acid value, the molar ratio of the urethane bond to the ureabond, and the gel fraction of the polyurethane polymer were measured bythe method described above. Table 2 shows the properties of thepolyurethane polymer dispersion PU-45.

TABLE 1 Preparation conditions for polyurethane (PU) polymer dispersionsPU Diol having acid polymer Polyisocyanate Polyol group Cross-linkerdispersion A B Number-average C D E No. Type (parts) Type (parts) Typemolecular weight (parts) Type (parts) Type (parts) PU-1 IPDI 32.0 HDI24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-2 MDI 37.5 HDI 24.0 PPG 2000106.0 DMPA 28.7 TMP 3.8 PU-3 IPDI 31.9 HDI 24.1 PPG 2000 108.5 DMBA 31.7TMP 3.8 PU-4 IPDI 32.9 HDI 24.9 PPG 2000 110.0 DMPA 28.7 Gly 3.8 PU-5IPDI 31.5 HDI 23.9 PPG 2000 112.1 DMPA 28.7 PE 3.8 PU-6 IPDI 31.5 HDI23.9 PPG 2000 112.1 DMPA 28.7 PPT 3.8 PU-7 IPDI 32.0 HDI 24.2 PPG 2000111.2 DMPA 28.7 TMP/Gly 2.2/1.6 PU-8 IPDI 32.0 HDI 24.2 PEG 2000 111.2DMPA 28.7 TMP 3.8 PU-9 IPDI 32.0 HDI 24.2 P(12BG) 2000 111.2 DMPA 28.7TMP 3.8 PU-10 IPDI 32.0 HDI 24.2 P(13BG) 2000 111.2 DMPA 28.7 TMP 3.8PU-11 IPDI 32.0 HDI 24.2 PTMG 2000 111.2 DMPA 28.7 TMP 3.8 PU-12 IPDI32.0 HDI 24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-13 IPDI 32.0 HDI 24.2PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-14 IPDI 32.0 HDI 24.2 PPG 2000 111.2DMPA 28.7 TMP 3.8 PU-15 IPDI 32.0 HDI 24.2 PPG 2000 111.2 DMPA 28.7 TMP3.8 PU-16 IPDI 32.0 HDI 24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-17 IPDI31.6 HDI 23.9 PPG 2000 112.8 DMPA 28.7 TMP 3.0 PU-18 IPDI 32.9 HDI 24.9PPG 2000 108.0 DMPA 28.7 TMP 5.7 PU-19 IPDI 30.3 HDI 22.9 PPG 2000 118.1DMPA 28.7 — — PU-20 IPDI 61.4 HDI 5.2 PPG 2000 100.9 DMPA 28.7 TMP 3.8PU-21 IPDI 7.0 HDI 47.4 PPG 2000 113.1 DMPA 28.7 TMP 3.8 PU-22 IPDI 57.7HDI 4.3 PPG 2000 105.5 DMPA 28.7 TMP 3.8 PU-23 IPDI 5.8 HDI 44.6 PPG2000 117.1 DMPA 28.7 TMP 3.8 PU-24 IPDI 68.1 — 0 PPG 2000 99.4 DMPA 28.7TMP 3.8 PU-25 IPDI 25.3 HDI 19.2 PPG 2000 132.6 DMPA 19.1 TMP 3.8 PU-26IPDI 27.0 HDI 20.4 PPG 2000 127.3 DMPA 21.5 TMP 3.8 PU-27 IPDI 45.5 HDI34.4 PPG 2000 68.5 DMPA 47.8 TMP 3.8 PU-28 IPDI 59.0 HDI 44.6 PPG 200025.7 DMPA 66.9 TMP 3.8 PU-29 IPDI 46.9 HDI 35.5 PPG 450 85.2 DMPA 28.7TMP 3.8 PU-30 IPDI 29.1 HDI 22.0 PPG 4000 116.4 DMPA 28.7 TMP 3.8 PU-31IPDI 32.0 HDI 24.2 PPG 2000 111.2 DMPA 28.7 TMP 3.8 PU-32 IPDI 23.6 HDI17.9 PPG 2000 137.9 DMPA 16.7 TMP 3.8 PU-33 IPDI 60.6 HDI 45.9 PPG 200020.4 DMPA 69.3 TMP 3.8 PU-34 IPDI 48.6 HDI 36.8 PPG 400 82.1 DMPA 28.7TMP 3.8 PU-35 IPDI 28.1 HDI 21.2 PPG 6000 118.2 DMPA 28.7 TMP 3.8 PU-36IPDI 68.1 — 0 PPG 2000 99.4 DMPA 28.7 TMP 3.8 PU-37 IPDI 32.0 HDI 24.2PC 2000 111.2 DMPA 28.7 TMP 3.8 PU-38 IPDI 32.0 HDI 24.2 PES 2000 111.2DMPA 28.7 TMP 3.8 PU-39 IPDI 32.0 HDI 24.2 PCL 2000 111.2 DMPA 28.7 TMP3.8 PU-40 IPDI 70.6 — — PPG 2000 100.8 DMPA 28.6 TMP/EDA 4.0/3.2

TABLE 2 Characteristics of polyurethane (PU) polymer dispersions Acidvalue (mgKOH/g) Percentage constituted by PU Acid value of PU unit(s)derived from HDI with polymer Acid value polymer resulting from Urethanerespect to all units derived Gel dispersion of PU unit(s) derived frombond/urea bond from polyisocyanate in PU fraction No. polymer diolhaving acid group (molar ratio) polymer (mol %) (%) PU-1 60 60 95.0/5.050 94 PU-2 60 60 95.0/5.0 50 94 PU-3 60 60 95.0/5.0 50 94 PU-4 60 6095.0/5.0 50 94 PU-5 60 60 95.0/5.0 50 94 PU-6 60 60 95.0/5.0 50 94 PU-760 60 95.0/5.0 50 94 PU-8 60 60 95.0/5.0 50 94 PU-9 60 60 95.0/5.0 50 94PU-10 60 60 95.0/5.0 50 94 PU-11 60 60 95.0/5.0 50 94 PU-12 60 6085.0/15.0 50 94 PU-13 60 60 90.0/10.0 50 94 PU-14 60 60 98.5/1.5 50 94PU-15 60 60 98.6/1.4 50 94 PU-16 60 60 100.0/0 50 94 PU-17 60 6095.0/5.0 50 88 PU-18 60 60 95.0/5.0 50 100 PU-19 60 60 95.0/5.0 50 81PU-20 60 60 95.0/5.0 10 94 PU-21 60 60 95.0/5.0 90 94 PU-22 60 6095.0/5.0 9 94 PU-23 60 60 95.0/5.0 91 94 PU-24 60 60 95.0/5.0 0 94 PU-2540 40 95.0/5.0 50 94 PU-26 45 45 95.0/5.0 50 94 PU-27 100 100 95.0/5.050 94 PU-28 140 140 95.0/5.0 50 94 PU-29 60 60 95.0/5.0 50 94 PU-30 6060 95.0/5.0 50 94 PU-31 60 60 84.0/16.0 50 94 PU-32 35 35 95.0/5.0 50 94PU-33 145 145 95.0/5.0 50 94 PU-34 60 60 95.0/5.0 50 94 PU-35 60 6095.0/5.0 50 94 PU-36 60 60 100.0/0 0 94 PU-37 60 60 95.0/5.0 50 94 PU-3860 60 95.0/5.0 50 94 PU-39 60 60 95.0/5.0 50 94 PU-40 60 60 84.0/16.0 094 PU-45 60 60 95.0/5.0 50 94Preparation of Pigment DispersionPreparation of Pigment Dispersion A

A commercially available Cab-O-Jet 400 (manufactured by Cabot Corp.) waswell stirred in water to yield a pigment dispersion A. Cab-O-Jet 400 isa self-dispersing carbon black pigment having a hydrophilic group on thesurface thereof. The pigment dispersion A had a pigment content (solidcontent) of 15.0% by mass and a pH of 9.0. The pigment had an averageparticle size of 130 nm.

Preparation of Pigment Dispersion B

1.5 g of 4-amino-1,2-benzenedicarboxylic acid was added to a solutioncontaining 5 g of concentrated hydrochloric acid dissolved in 5.5 g ofwater at five degrees Celsius. A vessel containing this solution wasplaced in an ice bath to maintain the solution at 10 degrees Celsius orless. 1.8 g of sodium nitrite dissolved in 9 g of water at five degreesCelsius was added to the solution. After the solution was stirred for 15minutes, 6 g of carbon black was added while stirring. The carbon blackhad a specific surface area of 220 m²/g and a DBP absorption of 105mL/100 g. After the solution was stirred for another 15 minutes, theresulting slurry was filtered through a filter paper (trade namestandard filter paper No. 2, manufactured by Advantec Toyo Kaisha,Ltd.). Particles on the filter were sufficiently washed with water. Theparticles were dried in an oven at 110 degrees Celsius to prepareself-dispersing carbon black. The self-dispersing carbon black wasdispersed in water at a pigment content of 10.0% by mass to prepare adispersion. This pigment dispersion contained self-dispersing carbonblack having a —C₆H₃—(COONa)₂ group on the surface thereof. Sodium ionsof the pigment dispersion were then substituted with potassium ions byan ion exchange method to prepare a pigment dispersion B. The pigmentdispersion B contained carbon black having a —C₆H₃—(COOK)₂ group on thesurface thereof. The pigment dispersion B had a pigment content (solidcontent) of 10.0% by mass and a pH of 8.0. The pigment had an averageparticle size of 80 nm.

Preparation of Pigment Dispersion C

500 g of carbon black having a specific surface area of 220 m²/g and aDBP absorption of 112 mL/100 g, 45 g ofaminophenyl(2-sulfoethyl)sulfone, and 900 g of distilled water in areactor were stirred at 300 rpm at a temperature of 55 degrees Celsiusfor 20 minutes. 40 g of 25% by mass sodium nitrite was then addeddropwise to the mixture for 15 minutes. After 50 g of distilled waterwas further added to the mixture, the mixture was allowed to react at 60degrees Celsius for two hours. The resulting product was removed fromthe reactor while being diluted with distilled water such that the solidcontent was 15.0% by mass. After centrifugation and purification toremove impurities, a dispersion (1) was obtained. Carbon black in thedispersion (1) had aminophenyl(2-sulfoethyl)sulfone functional groupsbonded to the surface thereof. The number of moles of functional groupsbonded to carbon black in the dispersion (1) was determined as describedbelow. The sodium ion of the dispersion (1) measured with a probe-typesodium electrode was converted into a value per mole of carbon blackpowder to determine the number of moles of functional groups bonded tocarbon black. The dispersion (1) was then added dropwise to apentaethylenehexamine solution for one hour with vigorous stirring whilethe temperature was maintained at room temperature. Thepentaethylenehexamine content ranged from 1 to 10 times the number ofmoles of sodium ions measured above. The amount of solution was equal tothe amount of dispersion (1). After this mixture was stirred for 18to 48hours, the mixture was purified to yield a dispersion (2) having a solidcontent of 10.0% by mass. Carbon black in the dispersion (2) hadpentaethylenehexamine bonded to the surface thereof.

190 g of a styrene-acrylic acid copolymer was weighed. Thestyrene-acrylic acid polymer had a weight-average molecular weight of8,000, an acid value of 140 mgKOH/g, and a polydispersity Mw/Mn of 1.5(wherein Mw denotes the weight-average molecular weight, and Mn denotesthe number-average molecular weight). 1,800 g of distilled water andpotassium hydroxide for neutralizing the polymer were added to dissolvethe styrene-acrylic acid copolymer while stirring, yielding an aqueousstyrene-acrylic acid copolymer solution. 500 g of the dispersion (2) wasthen added dropwise to the aqueous styrene-acrylic acid copolymersolution while stirring. This mixture of the dispersion (2) and theaqueous styrene-acrylic acid copolymer solution was transferred to anevaporating dish, was dried at 150 degrees Celsius for 15 hours, and wascooled to room temperature. The dried product was then dispersed indistilled water, which was adjusted to pH 9.0 with potassium hydroxidein advance, with a dispersing apparatus. 1.0 N aqueous potassiumhydroxide was added to the dispersion while stirring to adjust the pHwithin the range of 10 to 11. The dispersion was then desalted andpurified to remove impurities and coarse particles. Through theseprocedures, a pigment dispersion C containing polymer-attached carbonblack dispersed in water was prepared. The pigment dispersion C had apigment content (solid content) of 10.0% by mass and a pH of 10.1. Thepigment had an average particle size of 130 nm.

Preparation of Pigment Dispersion D

A styrene-acrylic acid copolymer having an acid value of 200 mgKOH/g anda weight-average molecular weight of 10,000 was neutralized with 10% bymass aqueous potassium hydroxide. 10 parts of carbon black having aspecific surface area of 210 m²/g and a DBP absorption of 74 mL/100 g,20 parts of the neutralized styrene-acrylic acid copolymer (solidcontent), and 70 parts of water were mixed. This mixture was dispersedwith a sand grinder for one hour, was centrifuged to remove coarseparticles, and was filtrated under pressure through a microfilter havinga pore size of 3.0 micrometers (manufactured by FUJIFILM Co.). Throughthese procedures, a pigment dispersion D in which carbon black wasdispersed in water by the polymer was prepared. The pigment dispersion Dhad a pigment content (solid content) of 10.0% by mass and a pH of 10.0.The pigment had an average particle size of 120 nm.

Preparation of Pigment Dispersion E

A pigment dispersion E in which carbon black was dispersed in water by apolyurethane polymer was prepared in the same manner as in the pigmentdispersion D except that the styrene-acrylic acid copolymer was replacedby the polyurethane polymer dispersion PU-1. The pigment dispersion Ehad a pigment content (solid content) of 10.0% by mass and a pH of 10.0.The pigment had an average particle size of 120 nm.

Preparation of Ink

A combination of the pigment dispersion, the polyurethane polymerdispersion, and a salt shown in Tables 3 and 4 was mixed with thefollowing components. The amount of ion-exchanged water (the remainder)was such that the total amount of the components of the ink was 100.0%by mass.

-   -   Pigment dispersion see Tables 3 and 4    -   Polyurethane polymer dispersion (polymer content (solid content)        was 20.0% by mass) see Tables 3 and 4    -   Salt see Tables 3 and 4    -   Glycerin 9.0% by mass    -   Diethylene glycol 5.0% by mass    -   Triethylene glycol 5.0% by mass    -   Acetylenol (trade name) E100 (surfactant, manufactured by        Kawaken Fine Chemicals Co., Ltd.) 0.1% by mass    -   Ion-exchanged water the remainder

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 micrometersunder pressure to prepare an ink.

TABLE 3 Preparation conditions for inks Pigment PU polymer dispersiondispersion PU polymer Salt Example Content Content content/pigmentContent No. No. (mass %) No. (mass %) content (times) Type (mass %)Example 1 A 20.00 PU-1 7.50 0.50 — 0 Example 2 B 30.00 PU-1 7.50 0.50 —0 Example 3 C 30.00 PU-1 7.50 0.50 — 0 Example 4 A 20.00 PU-2 7.50 0.50— 0 Example 5 A 20.00 PU-3 7.50 0.50 — 0 Example 6 A 20.00 PU-4 7.500.50 — 0 Example 7 A 20.00 PU-5 7.50 0.50 — 0 Example 8 A 20.00 PU-67.50 0.50 — 0 Example 9 A 20.00 PU-7 7.50 0.50 — 0 Example 10 A 20.00PU-8 7.50 0.50 — 0 Example 11 A 20.00 PU-9 7.50 0.50 — 0 Example 12 A20.00 PU-10 7.50 0.50 — 0 Example 13 A 20.00 PU-11 7.50 0.50 — 0 Example14 A 20.00 PU-12 7.50 0.50 — 0 Example 15 A 20.00 PU-13 7.50 0.50 — 0Example 16 A 20.00 PU-14 7.50 0.50 — 0 Example 17 A 20.00 PU-15 7.500.50 — 0 Example 18 A 20.00 PU-16 7.50 0.50 — 0 Example 19 A 20.00 PU-177.50 0.50 — 0 Example 20 A 20.00 PU-18 7.50 0.50 — 0 Example 21 A 20.00PU-19 7.50 0.50 — 0 Example 22 A 20.00 PU-20 7.50 0.50 — 0 Example 23 A20.00 PU-21 7.50 0.50 — 0 Example 24 A 20.00 PU-22 7.50 0.50 — 0 Example25 A 20.00 PU-23 7.50 0.50 — 0 Example 26 A 20.00 PU-24 7.50 0.50 — 0Example 27 A 20.00 PU-25 7.50 0.50 — 0 Example 28 A 20.00 PU-26 7.500.50 — 0

TABLE 4 Preparation conditions for inks Pigment PU polymer dispersiondispersion PU polymer Salt Content Content content/pigment ContentExample No. No. (mass %) No. (mass %) content (times) Type (mass %)Example 29 A 20.00 PU-27 7.50 0.50 — 0 Example 30 A 20.00 PU-28 7.500.50 — 0 Example 31 A 20.00 PU-29 7.50 0.50 — 0 Example 32 A 20.00 PU-307.50 0.50 — 0 Example 33 A 20.00 PU-1 7.50 0.50 Potassium phthalate 0.20Example 34 A 20.00 PU-1 7.50 0.50 Potassium benzoate 0.20 Example 35 A20.00 PU-1 7.50 0.50 Potassium sulfate 0.20 Example 36 A 20.00 PU-1 7.500.50 Potassium citrate 0.20 Example 37 A 20.00 PU-1 0.75 0.05 — 0Example 38 A 20.00 PU-1 30.00 2.00 — 0 Example 39 A 20.00 PU-1 0.60 0.04— 0 Example 40 A 20.00 PU-1 30.75 2.05 — 0 Example 41 A 20.00 PU-45 7.500.50 — 0 Comparative example 1 A 20.00 PU-31 7.50 0.50 — 0 Comparativeexample 2 A 20.00 PU-32 7.50 0.50 — 0 Comparative example 3 A 20.00PU-33 7.50 0.50 — 0 Comparative example 4 A 20.00 PU-34 7.50 0.50 — 0Comparative example 5 A 20.00 PU-35 7.50 0.50 — 0 Comparative example 6D 30.00 PU-36 7.50 0.50 — 0 Comparative example 7 A 20.00 PU-37 7.500.50 — 0 Comparative example 8 A 20.00 PU-38 7.50 0.50 — 0 Comparativeexample 9 A 20.00 PU-39 7.50 0.50 — 0 Comparative example 10 D 20.00PU-11 7.50 0.50 — 0 Comparative example 11 E 30.00 PU-1 7.50 0.50 — 0Comparative example 12 A 20.00 — 0 0 — 0 Comparative example 13 A 20.00— 0 0 Potassium phthalate 0.20 Reference example 1 D 30.00 PU-1 15.001.00 — 0 Reference example 2 D 30.00 PU-24 15.00 1.00 — 0 Referenceexample 3 A 20.00 PU-40 7.50 0.50 — 0

Polyurethane polymers and inks according to Comparative Examples 14 to20 were prepared as described below.

Comparative Example 14

An ink according to Comparative Example 14, was prepared as describedbelow with reference to Example 1 of PTL 4 (Japanese Patent Laid-OpenNo. 2004-285344). 35 parts of poly(tetramethylene ether) glycol, 3 partsof triethylene glycol, 16 parts of dimethylolpropionic acid, 9 parts oftetramethylenexylylene diisocyanate, and 25 parts of isophoronediisocyanate were allowed to react in acetone in a nitrogen stream toyield a urethane prepolymer. The urethane prepolymer was added dropwiseand dispersed in deionized water containing 12 parts of triethylamine.The acetone was removed under vacuum to yield a polyurethane polymerdispersion PU-41. PU-41 had a solid content of 30% by mass, aweight-average molecular weight of 20,840, and an acid value of 69mgKOH/g. 10.4 parts of a 25% by mass aqueous solution of a potassiumsalt of a styrene-acrylic acid copolymer Joncryl 683 (manufactured byJohnson Polymer; acid value 160 mgKOH/g, weight-average molecular weight8,000), 71.5 parts of ion-exchanged water, 5 parts of glycerin, 0.1parts of Proxel GXL(S) (manufactured by Avecia Ltd.), and 13 parts ofcarbon black MCF 88 (manufactured by Mitsubishi Chemical Corp.) weredispersed in a bead mill with 0.8-mm zirconia beads such that theaverage diameter of dispersed particles became 61.9 nm. Aftercentrifugation, coarse particles were removed by a 5-micrometer filterto yield a carbon black dispersion. The carbon black dispersion wasmixed with 26 parts of PU-41. 32.5 parts of triethylene glycol monobutylether, 32.5 parts of glycerin, 3.25 parts of Surfynol 465 (manufacturedby Air Products and Chemicals, Inc.), and 130.75 parts of ion-exchangedwater were added to the mixture to yield an ink according to ComparativeExample 14.

Comparative Example 15

An ink according to Comparative Example 15 was prepared as describedbelow with reference to Example 1 of PTL 2 (Japanese Patent Laid-OpenNo. 2008-179657). A four-neck flask equipped with a thermometer, anagitator, a nitrogen inlet, and a condenser tube was charged with 95 gof Placcel 205BA (manufactured by Daicel Chemical Industries, Ltd.), 11g of trimethylolpropane, 120 g of methyl ethyl ketone, and 0.54 g of1,4-diazabicyclo[2.2.2]octane. Placcel 205BA is acarboxylic-acid-modified polycaprolactone diol, in whichdimethylolbutanoic acid is modified with a lactone. After agitation for30 minutes, 74 g of isophorone diisocyanate was added to the four-neckflask. After agitation at room temperature in a nitrogen atmosphere forone hour, a reaction was performed at 70 degrees Celsius for four hours.After the reaction, the product was cooled to room temperature to yielda 60% by mass urethane prepolymer solution. 17.1 g of 50% by massaqueous potassium hydroxide and 350 g of ion-exchanged water were addedto 250 g of the urethane prepolymer solution in the four-neck flask andwere stirred at room temperature for 30 minutes. The mixture was heatedto 80 degrees Celsius in a nitrogen atmosphere and was subjected to achain extension reaction for two hours. After the reaction, methyl ethylketone and part of water were removed with a rotatory evaporator and anaspirator. Ion-exchanged water was then added such that the amount ofrecovery was 429 g to yield a 35% by mass polyurethane polymerdispersion PU-42. PU-42 had an acid value of 70 mgKOH/g and aweight-average molecular weight of 47,000.

After 3 kg of carbon black MA-100 (manufactured by Mitsubishi ChemicalCorp.) was mixed with 10 kg of water, the mixture was added to 4.5 kg ofa sodium hypochlorite solution (available chlorine concentration 12%).After agitation at a temperature in the range of 100 to 105 degreesCelsius for 10 hours, the resulting product was filtered. Dried wetcrystals were washed with water and were dried at 80 degrees Celsius toyield 2.5 kg of oxidized carbon black. The oxidized carbon black wasmixed with ion-exchanged water to prepare a slurry. The slurry wasdialyzed with a poly(methyl methacrylate) dialysis module FiltryzerB3-20A (manufactured by Toray Industries, Inc.) to remove sodium ionsand chloride ions in the oxidized carbon black and was dried to yielddialyzed oxidized carbon black. 120 g of the dialyzed oxidized carbonblack, 180 g of triethylene glycol monobutyl ether, and 700 g ofion-exchanged water were mixed in a Homo Disper agitator to prepare aslurry. A beaker containing the slurry was connected to acirculation-type bead mill DYNO-Mill KDL-A (manufactured by Willy A.Bachofen AG) through a tube. The slurry was dispersed with zirconiabeads having a diameter of 0.3 mm at 1,600 rpm for three hours toprepare an aqueous black pigment dispersion. 0.7 g of 50% by massaqueous potassium hydroxide, 0.5 g of ion-exchanged water, and 7.1 g ofthe polyurethane polymer dispersion PU-42 (solid content 35% by mass)were then added to 41.7 g of the aqueous black pigment dispersion toprepare an aqueous black pigment dispersion (pigment content: 10% bymass, polyurethane polymer content: 5% by mass). 25.8, g ofion-exchanged water, 3.5 g of glycerin, 1.7 g of 2-pyrrolidone, and 1.5g of ethylene glycol were then added to 17.5 g of the aqueous blackpigment dispersion to prepare an ink according to Comparative Example15.

Comparative Example 16

An ink according to Comparative Example 16 was prepared as describedbelow with reference to Example 1 of PTL 1 (PCT Japanese TranslationPatent Publication No. 2005-515289). 6.5 parts of self-dispersing carbonblack that was manufactured in accordance with International PublicationWO 01/94476 and was surface-treated by ozone oxidation, 0.1 parts of apolyurethane dispersion Hybridur 580 (manufactured by Air Products andChemicals, Inc.), 1.5 parts of a polyurethane dispersion Mace 85-302-1(manufactured by Mace Adhesives And Coatings Company Inc.), 9.5 parts ofglycerin, 6 parts of ethylene glycol, 0.8 parts of Surfynol 465(manufactured by Air Products and Chemicals, Inc.), and 75.6 parts ofwater were mixed to prepare an ink according to Comparative Example 16.

Comparative Example 17

An ink according to Comparative Example 17 was prepared as describedbelow with reference to Comparative Example 3 of PTL 3 (Japanese PatentLaid-Open No. 9-291242). 184.9 parts of poly(tetramethylene glycol)(molecular weight 3,000, hydroxyl value 38 mgKOH/g), 15.1 parts ofpoly(ethylene glycol) (molecular weight 2,000, hydroxyl value 56mgKOH/g), 21.5 parts of 2,2-dimethylolpropionic acid, and 200.0, partsof methyl ethyl ketone in a reactor equipped with a thermometer, anagitator, a reflux condenser tube, and a nitrogen gas-inlet pipe wereheated to 50 degrees Celsius in a nitrogen atmosphere while stirring.After the addition of 71.6 parts of isophorone diisocyanate, the mixturewas allowed to react at 80 degrees Celsius for 2.5 hours to yield anisocyanate-group-containing prepolymer solution. After cooling to 30degrees Celsius, a solution of 6.9 parts of propylene glycol and 229parts of methyl ethyl ketone was added dropwise and was allowed to reactat 70 degrees Celsius. A liquid mixture of 9.8 parts of 28% by massaqueous ammonia and 900 parts of water was then added dropwise. Theremoval of the solvent yielded a polyurethane polymer dispersion PU-43having no urea bond. PU-43 had an acid value of 30 mgKOH/g, a pH of 8.5,and a solid content of 25.0% by mass.

250.0 parts of an ethylene-acrylic acid copolymer Primacor 5983(manufactured by Dow Chemical Japan Ltd., acid value 156 mgKOH/g, meltindex 500 g/10 minutes) were neutralized with 278.6 parts of 10% by massaqueous sodium hydroxide and 471.4 parts of ion-exchanged water and werethen dissolved by heating to prepare an aqueous ethylene-acrylic acidcopolymer solution having a solid content of 25% by mass and a pH of9.0. 40.3 parts of the aqueous ethylene-acrylic acid copolymer solutionand 17.3 parts of the polyurethane polymer dispersion PU-43 were mixedto prepare a polymer composition. 15.0 parts of a phthalocyanine bluepigment Lionol Blue FG-7350 (manufactured by Toyo Ink Co., Ltd.), 0.3parts of a silicone antifoaming agent, 2.0 parts of a polyethylene waxdispersion (solid content 40% by mass), 4.5 parts of ethanol, and 20.6parts of ion-exchanged water were then added to 57.6 parts of thepolymer composition to prepare an ink according to Comparative Example17.

Comparative Example 18

An ink according to Comparative Example 18 was prepared as describedbelow with reference to Example 3 of Japanese Patent Laid-Open No.2003-342502. 250 g of a commercially available carbon black MA8(manufactured by Mitsubishi Chemical Corp.) was well dispersed in 1,000ml of ion-exchanged water. 1,000 g of ammonium peroxodisulfate was addedto the dispersion and was stirred at a temperature in the range of 60 to70 degrees Celsius for eight hours. The slurry was desalted with anultrafiltration membrane (manufactured by Toyo Roshi Kaisha, Ltd.)having a molecular weight cut-off of 10,000 such that the electricalconductivity of the filtrate was 0.5 ms/cm or less. The slurry wasconcentrated to a solid content of 20% by mass. 20 g of 25% by massaqueous potassium hydroxide was added dropwise to adjust the pH of theslurry to 8. Thus, a self-dispersing carbon black dispersion CB-1 havinga solid content of 15% by mass was prepared. The carbon black of CB-1had an average particle size of 85 nm, a polydispersity index of 0.15,and a zeta potential of −50 mV.

A self-dispersing carbon black dispersion CB-1, an acrylamide polymerSumirez Resin 7200A (manufactured by Sumika Chemtex Co., Ltd.), and apolyurethane polymer Superflex 150 (manufactured by Dai-ichi KogyoSeiyaku Co., Ltd.) were mixed for 10 minutes at a mass ratio of5.0:0.1:0.1 based on the solid content. 20% by mass of diethylene glycoland 0.10% by mass of Acetylenol (trade name) EH were then added to themixture. Water was then added to the mixture such that the carbon blackcontent (solid content) of the resulting ink was 5% by mass. The ink wasagitated for one hour. The ink was ultrafiltered through a 3-micrometermembrane filter (manufactured by Toyo Roshi Kaisha, Ltd.) to prepare anink according to Comparative Example 18.

Comparative Example 19

An ink according to Comparative Example 19 was prepared as describedbelow with reference to Example 4 of Japanese Patent Laid-Open No.2003-342502. An ink according to Comparative Example 19 was prepared inthe same manner as in Comparative Example 18 except that the acrylamidepolymer Sumirez Resin 7200A was replaced by Polysol Cogum 15H(manufactured by Showa Highpolymer Co., Ltd.) and the polyurethanepolymer Superflex 150 was replaced by a polyurethane polymer Superflex300.

Comparative Example 20

An ink according to Comparative Example 20 was prepared as describedbelow with reference to Example 1 of Japanese Patent Laid-Open No.2008-280363. 202, parts of a castor-oil-modified diol (manufactured byHokoku Corp., number-average molecular weight 732), 32 parts of acastor-oil-modified diol (manufactured by Hokoku Corp., number-averagemolecular weight 431), 79 parts of dimethylolpropionic acid, 43 parts ofpolyoxyethylene ether glycol PEG#600 (manufactured by NOF Corp.), 13parts of propylene glycol, 76 parts of 1,6-hexamethylene diisocyanate,155 parts of hydrogenated MDI (dicyclohexylmethane diisocyanate), and400 parts of methyl ethyl ketone in a reactor equipped with an agitator,a reflux condenser tube, a nitrogen inlet, and a thermometer were heatedto 75 degrees Celsius. After one hour, 0.3 parts of dibutyltin lauratewas added to the mixture. The mixture was kept at 75 degrees Celsius sothat the isocyanate group was decreased to 0.1% by mole or less. Thereaction product was then cooled to 45 degrees Celsius or less. 40 partsof 25% by mass aqueous ammonia and 1,400 parts of pure water were addedwhile stirring for phase inversion. Methyl ethyl ketone was removedunder vacuum while stirring at a temperature in the range of 20 to 60degrees Celsius to yield a polyurethane polymer dispersion PU-44 havinga solid content of 33% by mass and a pH of 6.7. 500 parts of aself-dispersing carbon black dispersion Aqua-Black 174 (manufactured byTokai Carbon Co., Ltd., solid content 20% by mass) was added to 3 partsof the polyurethane polymer dispersion PU-44 and was stirred at roomtemperature for two hours to prepare an ink according to ComparativeExample 20.

Evaluation 1

In the following evaluation, the criteria AAA to B refer to acceptablelevels, and the criteria C and D refer to unacceptable levels. Theevaluation was performed with an ink jet recording apparatus PIXUSiP3100 (manufactured by CANON KABUSHIKI KAISHA). The recordingconditions included a temperature of 23 degrees Celsius, a relativehumidity of 55%, and an ink droplet weight of 28 ng (within plus orminus 10%). With the ink jet recording apparatus, an image that wasrecorded under the conditions under which approximately 28 ng of one inkdroplet was applied to a unit area of 1/600 inches* 1/600 inches at aresolution of 600 dpi*600 dpi was assumed to have a print duty of 100%.

Optical Density 1

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 2 cm×2 cm solid image(print duty 100%) was printed on four recording media: a PPC sheetGF-500 (manufactured by CANON KABUSHIKI KAISHA), a PPC sheet 4024(manufactured by Xerox Co.), a PPC sheet Bright White (manufactured byHewlett-Packard Co.), and a PPC sheet Hammermill Jet Print (manufacturedby International Paper). After the solid image was left to stand for oneday, the optical density was measured with a reflection densitometerMacbeth RD-918 (manufactured by Macbeth). The following are evaluationcriteria for optical density. Tables 5 and 6 show the results.

AA: The average optical density of the four recording media was 1.40 ormore, and the highest optical density was 1.60 or more.

A: The average optical density of the four recording media was 1.40 ormore, but the highest optical density was 1.55 or more and less than1.60.

B: The average optical density of the four recording media was 1.40 ormore, but the highest optical density was less than 1.55.

C: The average optical density of the four recording media was less than1.40.

Scratch Resistance of Image 1

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 1.0 inch*0.5 inches solidimage (print duty 100%) was recorded on a PPC sheet GF-500 (manufacturedby CANON KABUSHIKI KAISHA). Ten minutes and one day after the recording,a Silbon paper and a weight having a contact pressure of 40 g/cm² wereplaced on the solid image, and the solid image and the Silbon paper wererubbed together. After the Silbon paper and the weight were removed,smudges on the solid image and a transfer to the white ground of theSilbon paper were visually inspected. The following are evaluationcriteria for the scratch resistance of an image. Tables 5 and 6 show theresults.

-   AAA: No smudge on the white ground was observed in the test after 10    minutes and the test after one day.-   AA: Little smudge on the white ground was observed in the test after    10 minutes, and no smudge on the white ground was observed in the    test after one day.-   A: Little smudge on the white ground was observed in the test after    10 minutes and the test after one day.-   B: Although unnoticeable smudges on the white ground were observed    in the test after 10 minutes, little smudge on the white ground was    observed in the test after one day.-   C: Smudges on the white ground were observed in the test after 10    minutes and the test after one day.-   D: Noticeable smudges on the white ground were observed in the test    after 10 minutes and the test after one day.    Highlighter Resistance of Image 1

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A vertical rule having awidth of 1/10 inches was recorded on a PPC sheet GF-500 (manufactured byCANON KABUSHIKI KAISHA). Five minutes and one day after the recording,the vertical rule was traced with a yellow highlighter OPTEX2(manufactured by ZEBRA Co., Ltd.), and immediately after that a line wasdrawn with the yellow highlighter on a white ground of a recordingmedium to check for contamination of the pen nib and contamination ofthe line on the white ground. The following are evaluation criteria forthe highlighter resistance of an image. Tables 5 and 6 show the results.

AAA: No contamination of the pen nib and no contamination of the line onthe white ground were observed in the test after five minutes and thetest after one day.

AA: In the test after five minutes, although coloring on the pen nib wasobserved, little contamination of the line on the white ground wasobserved. In the test after one day, there were no contamination of thepen nib and no contamination of the line on the white ground.

A: In the test after five minutes and the test after one day, althoughcoloring on the pen nib was observed, little contamination of the lineon the white ground was observed.

B: In the test after five minutes, although coloring on the pen nib wasobserved, the contamination of the line on the white ground wasunnoticeable. In the test after one day, although coloring on the pennib was observed, little contamination of the line on the white groundwas observed.

C: In the test after five minutes and the test after one day, there werecoloring of the pen nib and contamination of the line on the whiteground.

D: In the test after five minutes and the test after one day, there weresignificant coloring of the pen nib and significant contamination of theline on the white ground.

Ink Ejection Stability

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 19 cm*26 cm solid image(print duty 100%) was recorded on ten GF-500 PPC sheets (manufactured byCANON KABUSHIKI KAISHA). The solid image on the tenth sheet was visuallyinspected to evaluate ink ejection stability. The following areevaluation criteria for ink ejection stability. Tables 5 and 6 show theresults.

A: Neither white streak nor faint streak was observed, and the solidimage was properly recorded.

B: A few unnoticeable white streaks or faint streaks were observed.

C: Ink ejection was unstable, and white streaks or faint streaks wereobserved on the image.

Prevention of Image Deflection

An ink cartridge filled with the ink prepared as described above wasplaced in the ink jet recording apparatus. A 19 cm*26 cm solid image(print duty 100%) was recorded on two GF-500 PPC sheets (manufactured byCANON KABUSHIKI KAISHA). After the ink jet recording apparatus was leftto stand for 30 minutes, recording the image on two GF-500 PPC sheetswas performed ten times. A nozzle check pattern was then recorded withthe ink jet recording apparatus (PIXUS iP3100). The nozzle check patternwas visually inspected for image deflection. After the removal of arecording head, the face of the recording head was observed with amicroscope to check for face wetting. The following are evaluationcriteria for image deflection and face wetting. Tables 5 and 6 show theresults.

AA: No disorder in the nozzle check pattern was observed, indicating theprevention of image deflection. No face wetting was observed.

A: No disorder in the nozzle check pattern was observed, indicating theprevention of image deflection. Slight face wetting was observed.

B: An unnoticeable disorder in the nozzle check pattern was observed,indicating the prevention of image deflection. A little face wetting wasobserved.

C: A significant disorder in the nozzle check pattern was observed, andno satisfactory image was obtained, indicating the occurrence of imagedeflection. Face wetting was also observed.

TABLE 5 Evaluation results Evaluation of image Evaluation of ink ScratchHighlighter Prevention of Example No. Optical density resistanceresistance Ejection stability image deflection Example 1 A AAA AAA A AExample 2 A AAA AAA A A Example 3 A AAA AAA A A Example 4 A AAA AAA A AExample 5 A AAA AAA A A Example 6 A AAA AAA A A Example 7 A AAA AAA A AExample 8 A AAA AAA A A Example 9 A AAA AAA A A Example 10 A B B A AExample 11 A AAA AAA B A Example 12 A AAA AAA B A Example 13 A AAA AAA BA Example 14 A AAA AAA A B Example 15 A AAA AAA A A Example 16 A AAA AAAA A Example 17 A B B A A Example 18 A B B A A Example 19 A AAA AAA A AExample 20 A AAA AAA A A Example 21 A B B A A Example 22 A AAA AAA A AExample 23 A AAA AAA A A Example 24 A B B A A Example 25 A B B A AExample 26 A B B A A Example 27 A AAA AAA B A Example 28 A AAA AAA A AExample 29 A AAA AAA A A Example 30 A B B A A Example 31 A AAA AAA A AExample 32 A AAA AAA A A Example 33 AA AAA AAA A A Example 34 AA AAA AAAA A Example 35 AA AAA AAA A A Example 36 AA AAA AAA A A Example 37 A AAAAAA A A Example 38 A AAA AAA A A Example 39 A B B A A Example 40 A AAAAAA B A Example 41 A AAA AAA A A

TABLE 6 Evaluation results Evaluation of image Evaluation of ink OpticalScratch Highlighter Ejection Prevention of Example No density resistanceresistance stability image deflection Comparative example 1 A AAA AAA AC Comparative example 2 A AAA AAA C A Comparative example 3 A C C A AComparative example 4 A C C A A Comparative example 5 A C C A AComparative example 6 A C C A A Comparative example 7 A C C C AComparative example 8 A C C C A Comparative example 9 A C C C AComparative example 10 C AAA AAA C A Comparative example 11 C B C C AComparative example 12 A C C A A Comparative example 13 AA D D A AComparative example 14 C D D B B Comparative example 15 A D D C AComparative example 16 A C C C A Comparative example 17 C D D C AComparative example 18 B C C C C Comparative example 19 B C C C CComparative example 20 A C C C A Reference example 1 C AAA AAA A AReference example 2 C A A A AA Reference example 3 B AAA AAA B C

Three minutes after the recording, the results of the evaluation ScratchResistance of Image and Highlighter Resistance of Image for Example 41were better than the results for Example 1.

Preparation of Ink 1 and 2

Ink 1

The pigment dispersion A and the polyurethane polymer dispersion PU-1were mixed with other components as described below.

-   -   Pigment dispersion A (pigment content (solid content) was 10.0%        by mass) 30.0% by mass    -   Polyurethane polymer dispersion PU-1 (polymer content (solid        content) was 20.0% by mass) 7.5% by mass    -   Glycerin 9.0% by mass    -   Diethylene glycol 5.0% by mass    -   Triethylene glycol 5.0% by mass    -   Acetylenol (trade name) E100 (surfactant, manufactured by        Kawaken Fine Chemicals Co., Ltd.) 1.0% by mass    -   Ion-exchanged water 42.5% by mass

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 micrometersunder pressure to prepare an ink 1.

Ink 2

The pigment dispersion B and the polyurethane polymer dispersion PU-1were mixed with other components as described below.

-   -   Pigment dispersion B (pigment content (solid content) was 10.0%        by mass) 30.0% by mass    -   Polyurethane polymer dispersion PU-1 (polymer content (solid        content) was 20.0% by mass) 7.5% by mass    -   Glycerin 9.0% by mass    -   Diethylene glycol 5.0% by mass    -   Triethylene glycol 5.0% by mass    -   Acetylenol (trade name) E100 (surfactant, manufactured by        Kawaken Fine Chemicals Co., Ltd.) 1.0% by mass    -   Ion-exchanged water 42.5% by mass

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 micrometersunder pressure to prepare an ink 2.

Preparation of Liquid Composition

Liquid Composition 1

The following components were mixed. The amount of ion-exchanged water(the remainder) was such that the total amount of the components of theink was 100.0% by mass.

-   -   Reactant: calcium nitrate 5.0% by mass    -   Glycerin 5.0% by mass    -   1,5-pentanediol 5.0% by mass    -   Trimethylene glycol 7.0% by mass    -   Acetylenol (trade name) E100 (surfactant, manufactured by        Kawaken Fine Chemicals Co., Ltd.) 0.1% by mass    -   Ion-exchanged water the remainder

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 micrometersunder pressure to prepare a liquid composition 1.

Liquid Composition 2

A liquid composition 2 was prepared in the same manner as in the liquidcomposition 1 except that the reactant calcium nitrate was replaced byaluminum nitrate.

Liquid Composition 3

A liquid composition 3 was prepared in the same manner as in the liquidcomposition 1 except that the reactant calcium nitrate was replaced byyttrium nitrate.

Liquid Composition 4

A liquid composition 4 was prepared in the same manner as in the liquidcomposition 1 except that the reactant calcium nitrate was replaced bymagnesium nitrate.

Liquid Composition 5

The following components were mixed. The amount of ion-exchanged water(the remainder) was such that the total amount of the components of theink was 100.0% by mass.

-   -   Reactant: sodium citrate 10.0% by mass    -   Glycerin 5.0% by mass    -   1,5-pentanediol 5.0% by mass    -   Trimethylolpropane 7.0% by mass    -   NIKKOL BC-20 (surfactant, manufactured by Nikko Chemicals Co.,        Ltd.) 1.0% by mass    -   Ion-exchanged water the remainder

The mixture was well dispersed and was passed through a microfilter(manufactured by Fujifilm Corp.) having a pore size of 3.0 micrometersunder pressure. The mixture was then treated with sulfuric acid toprepare a liquid composition 5 having a pH of 4.0.

Liquid Composition 6

A liquid composition 6 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 3.0.

Liquid Composition 7

A liquid composition 7 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 3.5.

Liquid Composition 8

A liquid composition 8 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 5.5.

Liquid Composition 9

A liquid composition 9 was prepared in the same manner as in the liquidcomposition 5 except that the pH was 6.0.

The absorbances of the liquid compositions 1 to 9 without dilution weremeasured with a Hitachi double-beam spectrophotometer U-2900(manufactured by Hitachi High-Technologies Corp.). Any of the liquidcompositions had a ratio Amax/Amin of the maximum absorbance Amax to theminimum absorbance Amin of 1.0 or more and 2.0 or less at a wavelengthin the range of 400 to 780 nm.

Evaluation 2

In the following evaluation, the criteria AAA to B refer to acceptablelevels, and the criteria C and D refer to unacceptable levels. TheOptical Density, Scratch Resistance of Image, and Highlighter Resistanceof Image were evaluated with an ink jet recording apparatus PIXUSPro9500 (manufactured by CANON KABUSHIKI KAISHA). The recordingconditions included a temperature of 23 degrees Celsius and a relativehumidity of 55%. An ink cartridge separately filled with the ink and theliquid composition listed in Table 7 was placed in the ink jet recordingapparatus PIXUS Pro9500. The ink was contained in a chamber for yellow,and the liquid composition was contained in a chamber for gray.Recording was performed unidirectionally from the home position to theopposite position. The width of the recording corresponded to the nozzlewidth of the head. After the liquid composition was applied, the ink wasapplied on the liquid composition in the same pass to form an image.With the ink jet recording apparatus, an image recorded under theconditions under which approximately 16 ng of one ink droplet wasapplied to a unit area (one pixel) of 1/600 inches* 1/600 inches at aresolution of 600 dpi*600 dpi was assumed to have an ink print duty of100%. An image recorded under the conditions under which approximately 7ng of one droplet of the liquid composition was applied to one pixel wasassumed to have a liquid composition print duty of 100%.

Optical Density 2

An ink cartridge filled with the ink and the liquid composition preparedas described above was placed in the ink jet recording apparatus. A 2cm*2 cm solid image (ink and liquid composition print duties 100%) wasprinted on four types of recording media: a PPC sheet GF-500(manufactured by CANON KABUSHIKI KAISHA), a PPC sheet 4024 (manufacturedby Xerox Corp.), a PPC sheet Bright White (manufactured byHewlett-Packard Co.), and a PPC sheet Hammermill Jet Print (manufacturedby International Paper). After the solid image was left to stand for oneday, the optical density was measured with a reflection densitometerMacbeth RD-918 (manufactured by Macbeth). The following are evaluationcriteria for optical density. Table 7 shows the results.

AA: The average optical density of the four recording media was 1.40 ormore, and the highest optical density was 1.60 or more.

A: The average optical density of the four recording media was 1.40 ormore, but the highest optical density was 1.55 or more and less than1.60.

B: The average optical density of the four recording media was 1.40 ormore, but the highest optical density was less than 1.55.

C: The average optical density of the four recording media was less than1.40.

Scratch Resistance of Image 2

An ink cartridge filled with the ink and the liquid composition preparedas described above was placed in the ink jet recording apparatus. A 1.0inch*0.5 inches solid image (ink and liquid composition print duties100%) was recorded on a PPC sheet GF-500 (manufactured by CANONKABUSHIKI KAISHA).

(1) Evaluation of Scratch Resistance of Image Three Minutes AfterRecording

Three minutes after the recording, a Silbon paper and a weight having acontact pressure of 40 g/cm² were placed on the solid image, and thesolid image and the Silbon paper were rubbed together. After the Silbonpaper and the weight were removed, smudges on the solid image and atransfer to the white ground of the Silbon paper were visuallyinspected. The following are evaluation criteria for the scratchresistance of an image. Table 7 shows the results.

AA: No smudge on the white ground was observed in the test after threeminutes.

A: Little smudge on the white ground was observed in the test afterthree minutes.

B: Unnoticeable smudges on the white ground were observed in the testafter three minutes.

C: Smudges on the white ground were observed in the test after threeminutes.

D: Noticeable smudges on the white ground were observed in the testafter three minutes.

(2) Evaluation of Scratch Resistance of Image 10 Minutes and One DayAfter Recording

Ten minutes and one day after the recording, a Silbon paper and a weighthaving a contact pressure of 40 g/cm² were placed on the solid image,and the solid image and the Silbon paper were rubbed together. After theSilbon paper and the weight were removed, smudges on the solid image anda transfer to the white ground of the Silbon paper were visuallyinspected. The following are evaluation criteria for the scratchresistance of an image. Table 7 shows the results.

AAA: No smudge on the white ground was observed in the test after 10minutes and the test after one day.

AA: Little smudge on the white ground was observed in the test after 10minutes, and no smudge on the white ground was observed in the testafter one day.

A: Little smudge on the white ground was observed in the test after 10minutes and the test after one day.

B: Although unnoticeable smudges on the white ground were observed inthe test after 10 minutes, little smudge on the white ground wasobserved in the test after one day.

C: Smudges on the white ground were observed in the test after 10minutes and the test after one day.

D: Noticeable smudges on the white ground were observed in the testafter 10 minutes and the test after one day.

Highlighter Resistance of Image 2

An ink cartridge filled with the ink and the liquid composition preparedas described above was placed in the ink jet recording apparatus. Avertical rule having a width of 1/10 inches was recorded on a PPC sheetGF-500 (manufactured by CANON KABUSHIKI KAISHA).

(1) Evaluation of Highlighter Resistance of Image Three Minutes AfterRecording

Three minutes after the recording, the vertical rule was traced with ayellow highlighter OPTEX2 (manufactured by ZEBRA Co., Ltd.), andimmediately after that a line was drawn with the yellow highlighter on awhite ground of a recording medium to check for contamination of the pennib and contamination of the line on the white ground. The following areevaluation criteria for the highlighter resistance of an image. Table 7shows the results.

AA: No contamination of the pen nib and no contamination of the line onthe white ground were observed in the test after three minutes.

A: In the test after three minutes, although coloring on the pen nib wasobserved, little contamination of the line on the white ground wasobserved.

B: In the test after three minutes, although coloring on the pen nib wasobserved, the contamination of the line on the white ground wasunnoticeable.

C: In the test after three minutes, there were coloring of the pen niband contamination of the line on the white ground.

D: In the test after three minutes, there were significant coloring ofthe pen nib and significant contamination of the line on the whiteground.

(2) Evaluation of Highlighter Resistance of Image Five Minutes and OneDay After Recording

Five minutes and one day after the recording, the vertical rule wastraced with a yellow highlighter OPTEX2 (manufactured by ZEBRA Co.,Ltd.), and immediately after that a line was drawn with the yellowhighlighter on a white ground of a recording medium to check forcontamination of the pen nib and contamination of the line on the whiteground. The following are evaluation criteria for the highlighterresistance of an image. Table 7 shows the results.

-   AAA: No contamination of the pen nib and no contamination of the    line on the white ground were observed in the test after five    minutes and the test after one day.-   AA: In the test after five minutes, although coloring on the pen nib    was observed, little contamination of the line on the white ground    was observed. In the test after one day, there were no contamination    of the pen nib and no contamination of the line on the white ground.-   A: In the test after five minutes and the test after one day,    although coloring on the pen nib was observed, little contamination    of the line on the white ground was observed.-   B: In the test after five minutes, although coloring on the pen nib    was observed, the contamination of the line on the white ground was    unnoticeable. In the test after one day, although coloring on the    pen nib was observed, little contamination of the line on the white    ground was observed.-   C: In the test after five minutes and the test after one day, there    were coloring of the pen nib and contamination of the line on the    white ground.-   D: In the test after five minutes and the test after one day, there    were significant coloring of the pen nib and significant    contamination of the line on the white ground.

TABLE 7 Combination of liquid composition and ink and evaluation resultsEvaluation of image Scratch resistance Highlighter resistance (1) 3 min(2) 10 min and (1) 3 min (2) 5 min and Liquid composition Ink Opticalafter 1 day after after 1 day after Example No. No. density recordingrecording recording recording Example 42 Liquid composition 1 Ink 1 AAAAA AA AAA AA Example 43 Liquid composition 1 Ink 2 AA AAA AA AAA AAExample 44 Liquid composition 2 Ink 1 AA AAA AA AAA AA Example 45 Liquidcomposition 3 Ink 1 AA AAA AA AAA AA Example 46 Liquid composition 4 Ink1 AA AAA A AAA A Example 47 Liquid composition 5 Ink 1 AA AAA AA AAA AAExample 48 Liquid composition 6 Ink 1 AA AAA AA AAA AA Example 49 Liquidcomposition 7 Ink 1 AA AAA AA AAA AA Example 50 Liquid composition 8 Ink1 AA AAA AA AAA AA Example 51 Liquid composition 9 Ink 1 AA AAA B AAA B

When the liquid composition had a pH of less than 3.5, as in Example 48,the wettability of a member in contact with the liquid composition wasinferior to the liquid composition having a pH of 3.5 or more (forexample, Example 49).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-075884, filed Mar. 30, 2011, and Japanese Patent Application No.2011-075885, filed Mar. 30, 2011, which are hereby incorporated byreference herein in their entirety.

The invention claimed is:
 1. An ink jet ink, comprising: a polyurethanepolymer and a self-dispersing pigment, the polyurethane polymer havingunits derived from a polyisocyanate, a polyol having no acid group, anda diol having an acid group, wherein the polyol having no acid groupcontains a polyether polyol having no acid group, and the percentage (%by mole) constituted by a unit derived from the polyether polyol havingno acid group with respect to all the units derived from the polyolhaving no acid group is 80% by mole or more, the polyether polyol has anumber-average molecular weight of 450 or more to 4,000 or less, theratio of the percentage (% by mole) of urethane bonds in thepolyurethane polymer to the percentage (% by mole) of urea bonds in thepolyurethane polymer is 85.0/15.0 or more to 100.0/0 or less, and thediol having an acid group is at least one selected fromdimethylolpropionic acid and dimethylolbutanoic acid, and the acid valueof the polyurethane polymer resulting from the unit(s) derived from thediol having an acid group is 40 mgKOH/g or more to 140 mgKOH/g or less.2. The ink jet ink according to claim 1, wherein the polyisocyanatecontains a hexamethylene diisocyanate, and the percentage (% by mole)constituted by the unit(s) derived from the hexamethylene diisocyanatewith respect to all the units derived from the polyisocyanate in thepolyurethane polymer is 10% by mole or more to 90% by mole or less. 3.The ink jet ink according to claim 1, wherein the polyurethane polymeris a cross-linked polyurethane polymer.
 4. The ink jet ink according toclaim 1, wherein the ratio of the percentage (% by mole) of urethanebonds in the polyurethane polymer to the percentage (% by mole) of ureabonds in the polyurethane polymer is 85.0/15.0 or more to 98.5/1.5 orless.
 5. The ink jet ink according to claim 1, wherein the polyetherpolyol having no acid group contains at least one selected frompoly(ethylene glycol), poly(propylene glycol), poly(1,2-butyleneglycol), and poly(1,3-butylene glycol).
 6. The ink jet ink according toclaim 1, wherein the polyether polyol having no acid group containspoly(propylene glycol).
 7. The ink jet ink according to claim 1, whereinthe ratio of the polyurethane polymer content (% by mass) of the ink tothe self-dispersing pigment content (% by mass) based on the total massof the ink is 0.05 or more to 2.00 or less.
 8. The ink jet ink accordingto claim 1, further comprising a salt.
 9. The ink jet ink according toclaim 8, wherein the salt contains at least one anion selected from Cl⁻,Br⁻, I⁻, ClO⁻, ClO²⁻, ClO³⁻, ClO⁴⁻, NO²⁻, NO³⁻, SO₄ ²⁻, CO₃ ²⁻, HCOO⁻,CH₃COO⁻, C₂H₄(COO⁻)₂, C₆H₅COO⁻, and C₆H₄(COO⁻)₂.
 10. An ink cartridge,comprising an ink storage portion configured to store an ink, whereinthe ink is an ink jet ink according to claim
 1. 11. An ink jet recordingmethod, comprising ejecting an ink by an ink jet method, wherein the inkis an ink jet ink according to claim
 1. 12. An ink jet recording method,comprising: ejecting an ink containing a pigment and a polyurethanepolymer through an ink jet recording head onto a recording medium; andapplying a liquid composition to the recording medium such that theliquid composition at least partly overlaps the ink, the liquidcomposition destabilizing the dispersion of the pigment in the ink anddecreasing the solubility of the polyurethane polymer, wherein the inkis an ink jet ink according to claim 1.